diff --git a/README.md b/README.md
index f5bad26..75f07bf 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Eine browserbasierte Tabellenkalkulation nach dem Vorbild von **Apple Numbers**.
 ```bash
 npm install
 npm run dev        # Vite-Devserver
-npm run test       # Vitest-Suite (Engine + Store + IO), 528 Tests
+npm run test       # Vitest-Suite (Engine + Store + IO), 553 Tests
 npm run build      # Typecheck + Produktionsbuild
 npm run lint       # ESLint (flat config)
 ```
@@ -878,7 +878,10 @@ das Dropdown, ohne die Bearbeitung selbst abzubrechen.
 `MIN`, `MAX`, `COUNT`, `CONCAT` (Alias `CONCATENATE`), `IF(Bedingung; Dann;
 Sonst?)`, `AND(Bedingung1; Bedingung2; …)`, `OR(Bedingung1; Bedingung2; …)`,
 `NOT(Bedingung)`, `DATE(Jahr; Monat; Tag)`, `TIME(Std; Min; Sek)`, `TODAY()`,
-`NOW()`, `XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; [WennNichtGefunden];
+`NOW()`, die Datums-/Zeitfunktionen `YEAR`, `MONTH`, `DAY`, `HOUR`,
+`MINUTE`, `SECOND`, `WEEKDAY`, `ISOWEEKNUM`, `WEEKNUM`, `EDATE`, `EOMONTH`,
+`DAYS`, `DATEDIF`, `NETWORKDAYS`, `WORKDAY`, `DATEVALUE`, `TIMEVALUE` (s. „Datum
+& Zeit" unten), `XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; [WennNichtGefunden];
 [Vergleichsmodus]; [Suchmodus])` — plus alle über den **„fx"-Button** angelegten
 benutzerdefinierten Funktionen (siehe „Benutzerdefinierte Funktionen" oben),
 die sich für den Aufrufer identisch verhalten. Argumente gemischt:
@@ -922,7 +925,8 @@ ohne Zusatzaufwand: `=fällig - rechnungsdatum` zählt Tage, `=rechnungsdatum
   und sofort mit dem passenden Format angezeigt — genau wie in Excel/Numbers.
   Erkannt werden ISO (`2026-09-02`, auch mit `14:30`/`14:30:15` angehängt)
   und Punkt-Daten (`02.09.2026`, `02.09.26`, ebenfalls mit Uhrzeit), sowie
-  reine Uhrzeiten (`14:30`). **Absichtlich nicht** erkannt: Schrägstrich-Daten
+  reine Uhrzeiten (`14:30`); ab 24 Stunden (`37:30`) wird daraus eine
+  **Dauer**. **Absichtlich nicht** erkannt: Schrägstrich-Daten
   (`02/09/2026`) — Tag-zuerst vs. Monat-zuerst ist über die unterstützten
   Locales (en-US ist monatzuerst, alle anderen tagzuerst) nicht eindeutig
   auflösbar; falsches Raten würde das Datum lautlos verfälschen, deshalb
@@ -932,13 +936,39 @@ ohne Zusatzaufwand: `=fällig - rechnungsdatum` zählt Tage, `=rechnungsdatum
   Datums-/Zeitwert (Überlauf wie in Excel erlaubt: `DATE(2026;13;1)` =
   01.01.2027, `TIME(25;0;0)` = 01:00 des Folgetags-Anteils); `TODAY()` /
   `NOW()` liefern das aktuelle Datum bzw. Datum+Uhrzeit. Formel-Ergebnisse
-  bekommen **kein** automatisches Zellformat (wie bei jeder anderen Formel
-  auch) — das Zahlenformat-Dropdown auf „Datum"/„Zeit"/„Datum + Zeit" stellen.
+  bekommen **kein** automatisches Zellformat — das Zahlenformat-Dropdown auf
+  „Datum"/„Zeit"/„Datum + Zeit" stellen. Einzige Ausnahme: eine Formel, deren
+  Zahlen-Eingaben **alle Zeiten oder Dauern** sind (`=SUM(B2:B8)` über
+  Arbeitszeiten, `=C2-B2` für Ende − Beginn), zeigt ihr Ergebnis als
+  **Dauer**, solange die Zelle auf „Automatisch" steht.
   `TODAY`/`NOW` sind **nicht volatil**: diese Engine rechnet rein
   vorgänger-getrieben neu, ohne Timer — eine Zelle mit `=TODAY()`
   aktualisiert sich erst bei ihrer eigenen nächsten Neuberechnung, nicht von
   selbst beim Tageswechsel (siehe „Bekannte Grenzen" unten).
 
+Das Zahlenformat **„Dauer"** zählt Stunden über 24 hinaus weiter (`37:30`
+statt `13:30`) — für Summen von Arbeits- oder Laufzeiten; Sekunden
+erscheinen nur, wenn welche da sind (`100:00:30`). **„Zeit"** bleibt eine
+Uhrzeit und springt nach 24 Stunden wieder auf `00:00`. **„Datum + Zeit"**
+zeigt bei einer reinen Uhrzeit (unter einem Tag) nur die Uhrzeit statt des
+Nullpunkts 30.12.1899.
+
+**Datums- und Zeitfunktionen**: `YEAR`/`MONTH`/`DAY`/`HOUR`/`MINUTE`/`SECOND`
+lesen einen Teil aus (auf ganze Sekunden gerundet). Wo Excel eine
+US-Voreinstellung hat, gilt hier **ISO 8601**: `WEEKDAY(Datum; [Typ])`
+zählt Montag = 1 … Sonntag = 7 (Typ 1: Sonntag = 1, Typ 3: Montag = 0),
+`ISOWEEKNUM` und `WEEKNUM(Datum; [Typ])` liefern die ISO-Kalenderwoche
+(`WEEKNUM` Typ 1/2: US-Zählung mit Wochenbeginn Sonntag/Montag).
+`EDATE(Datum; Monate)` verschiebt um Monate (31.01. + 1 = 28./29.02.),
+`EOMONTH(Datum; Monate)` liefert den Monatsletzten, `DAYS(Ende; Start)` die
+Kalendertage dazwischen, `DATEDIF(Start; Ende; Einheit)` den Abstand in
+`"Y"`/`"M"`/`"D"` bzw. als Rest `"YM"`/`"MD"`/`"YD"`.
+`NETWORKDAYS(Start; Ende; [Feiertage])` zählt Arbeitstage (Mo–Fr, beide
+Enden eingeschlossen), `WORKDAY(Start; Tage; [Feiertage])` rechnet eine
+Frist in Arbeitstagen — Feiertage als Zellen/Bereich. `DATEVALUE(Text)` und
+`TIMEVALUE(Text)` lesen Datum bzw. Uhrzeit aus einem Text in jedem Format,
+das auch beim Eintippen erkannt wird. Ungültige Argumente ergeben `#VALUE!`.
+
 Das Zahlenformat **„Datum (ISO)"** zeigt ein Datum immer als `JJJJ-MM-TT`
 (`2026-09-02`) — unabhängig von der aktiven Locale (Blatt/Dokument oder
 Zelle), anders als „Datum", das der jeweiligen Locale folgt (`02.09.2026`
@@ -1082,7 +1112,7 @@ Grundrechenarten: `Übersicht!B2 = =SUM(Umsatz!B2:E2)`, `D2 = =B2-C2`,
 
 ## Tests
 
-`npm run test` — Vitest, 528 Tests:
+`npm run test` — Vitest, 553 Tests:
 
 - **lexer / parser** — Präzedenz & Assoziativität (`^` vor `*/` vor `+-`, `^`
   rechts, `-2^2 = -(2^2)`), lokale/qualifizierte/quoted Bezüge, `!` **und** `::`,
@@ -1404,7 +1434,10 @@ Grundrechenarten: `Übersicht!B2 = =SUM(Umsatz!B2:E2)`, `D2 = =B2-C2`,
   und Punkt-Daten, absichtlich **nicht** Schrägstrich-Daten (Tag-zuerst vs.
   Monat-zuerst ist über die unterstützten Locales nicht eindeutig auflösbar,
   s. o.). Formel-Ergebnisse (`=DATE(...)`, `=TODAY()`, `=A1+30`) bekommen kein
-  automatisches Zellformat — wie jedes andere Formelergebnis auch. `TODAY()`/
+  automatisches Zellformat — außer „Dauer" für Formeln über reine Zeiten.
+  Das gilt auch dann, wenn die Formel die Zeiten in Stunden umrechnet
+  (`=A1*24`); dort das Format „Automatisch" durch „Ohne Formatierung" o. ä.
+  ersetzen. `TODAY()`/
   `NOW()` sind nicht volatil: sie aktualisieren sich erst bei der nächsten
   Neuberechnung ihrer eigenen Zelle, nicht spontan mit der Zeit.
 - **XLOOKUP** bewusst ohne: Näherungsvergleich (`Vergleichsmodus` -1/1) auf
diff --git a/src/data/codeReference.ts b/src/data/codeReference.ts
index 582ff51..292ef8b 100644
--- a/src/data/codeReference.ts
+++ b/src/data/codeReference.ts
@@ -435,6 +435,23 @@ const FUNCTION_DESCRIPTIONS: Record<string, string> = {
   TODAY: 'Heutiges Datum.',
   NOW: 'Aktuelles Datum mit Uhrzeit.',
   XLOOKUP: 'Sucht einen Wert und gibt den passenden Eintrag aus einem zweiten Bereich zurück.',
+  YEAR: 'Jahr eines Datums.',
+  MONTH: 'Monat eines Datums (1–12).',
+  DAY: 'Tag des Monats (1–31).',
+  HOUR: 'Stunde einer Uhrzeit (0–23).',
+  MINUTE: 'Minute einer Uhrzeit (0–59).',
+  SECOND: 'Sekunde einer Uhrzeit (0–59).',
+  WEEKDAY: 'Wochentag nach ISO: Montag = 1 … Sonntag = 7. Typ 1: Sonntag = 1, Typ 3: Montag = 0.',
+  ISOWEEKNUM: 'Kalenderwoche nach ISO 8601 (Woche 1 enthält den ersten Donnerstag des Jahres).',
+  WEEKNUM: 'Kalenderwoche, standardmäßig nach ISO. Typ 1/2: US-Zählung mit Wochenbeginn Sonntag/Montag.',
+  EDATE: 'Gleicher Tag einige Monate später (negativ: früher), am Monatsende gekürzt.',
+  EOMONTH: 'Letzter Tag des Monats, einige Monate vor oder nach dem Datum.',
+  DAYS: 'Anzahl Kalendertage zwischen zwei Daten.',
+  DATEDIF: 'Abstand zweier Daten in "Y" Jahren, "M" Monaten, "D" Tagen oder als Rest "YM", "MD", "YD".',
+  NETWORKDAYS: 'Arbeitstage (Mo–Fr) zwischen zwei Daten, beide eingeschlossen, ohne Feiertage.',
+  WORKDAY: 'Datum nach einer Anzahl Arbeitstage (Mo–Fr), Feiertage werden übersprungen.',
+  DATEVALUE: 'Datum aus einem Text wie "2026-09-02" oder "02.09.2026".',
+  TIMEVALUE: 'Uhrzeit aus einem Text wie "14:30".',
 };
 
 /** German search terms for built-ins whose description doesn't already say it. */
@@ -451,6 +468,23 @@ const FUNCTION_KEYWORDS: Record<string, string> = {
   TODAY: 'heute datum',
   NOW: 'jetzt zeit',
   XLOOKUP: 'suchen sverweis verweis nachschlagen',
+  YEAR: 'jahr datum',
+  MONTH: 'monat datum',
+  DAY: 'tag datum',
+  HOUR: 'stunde zeit uhrzeit',
+  MINUTE: 'minute zeit uhrzeit',
+  SECOND: 'sekunde zeit uhrzeit',
+  WEEKDAY: 'wochentag tag woche',
+  ISOWEEKNUM: 'kalenderwoche kw woche',
+  WEEKNUM: 'kalenderwoche kw woche',
+  EDATE: 'monat addieren datum',
+  EOMONTH: 'monatsende ultimo',
+  DAYS: 'tage differenz abstand',
+  DATEDIF: 'differenz abstand alter jahre monate tage',
+  NETWORKDAYS: 'arbeitstage werktage feiertage',
+  WORKDAY: 'arbeitstag werktag feiertage frist',
+  DATEVALUE: 'datum text umwandeln',
+  TIMEVALUE: 'zeit uhrzeit text umwandeln',
 };
 
 const FUNCTIONS: RefEntry[] = BUILTIN_SIGNATURES.map((s) => ({
diff --git a/src/engine/__tests__/dateFunctions.spec.ts b/src/engine/__tests__/dateFunctions.spec.ts
new file mode 100644
index 0000000..2ae6cce
--- /dev/null
+++ b/src/engine/__tests__/dateFunctions.spec.ts
@@ -0,0 +1,125 @@
+import { describe, it, expect } from 'vitest';
+import { FUNCTIONS } from '../functions';
+import { dateToSerial, timeSerial } from '../dates';
+import { isError, type CellValue } from '../types';
+
+const call = (name: string, ...args: CellValue[]) => FUNCTIONS[name](args);
+const d = (y: number, m: number, day: number) => dateToSerial(y, m, day);
+
+describe('date parts', () => {
+  const t = dateToSerial(2026, 9, 2, 14, 30, 15);
+
+  it('YEAR / MONTH / DAY / HOUR / MINUTE / SECOND', () => {
+    expect(call('YEAR', t)).toBe(2026);
+    expect(call('MONTH', t)).toBe(9);
+    expect(call('DAY', t)).toBe(2);
+    expect(call('HOUR', t)).toBe(14);
+    expect(call('MINUTE', t)).toBe(30);
+    expect(call('SECOND', t)).toBe(15);
+  });
+
+  it('rounds to the whole second, so float noise never shows as :59', () => {
+    const noisy = timeSerial(14, 29, 59) + 1 / 86400;
+    expect(call('HOUR', noisy)).toBe(14);
+    expect(call('MINUTE', noisy)).toBe(30);
+    expect(call('SECOND', noisy)).toBe(0);
+  });
+
+  it('checks argument count and type', () => {
+    expect(isError(call('YEAR'))).toBe(true);
+    expect(isError(call('YEAR', 1, 2))).toBe(true);
+    expect(isError(call('YEAR', 'kein Datum'))).toBe(true);
+  });
+});
+
+describe('WEEKDAY (ISO default: Monday = 1)', () => {
+  const wed = d(2026, 9, 2);
+  it('defaults to ISO numbering', () => {
+    expect(call('WEEKDAY', wed)).toBe(3);
+    expect(call('WEEKDAY', d(2026, 9, 6))).toBe(7); // Sunday
+  });
+  it('supports Excel types 1, 2, 3', () => {
+    expect(call('WEEKDAY', wed, 1)).toBe(4); // Sunday = 1
+    expect(call('WEEKDAY', wed, 2)).toBe(3);
+    expect(call('WEEKDAY', wed, 3)).toBe(2); // Monday = 0
+    expect(isError(call('WEEKDAY', wed, 7))).toBe(true);
+  });
+});
+
+describe('ISO week numbers', () => {
+  it('handles the year boundaries', () => {
+    expect(call('ISOWEEKNUM', d(2026, 1, 1))).toBe(1); // Thursday
+    expect(call('ISOWEEKNUM', d(2027, 1, 1))).toBe(53); // Friday -> last week of 2026
+    expect(call('ISOWEEKNUM', d(2021, 1, 3))).toBe(53); // Sunday -> week 53 of 2020
+    expect(call('ISOWEEKNUM', d(2024, 12, 30))).toBe(1); // Monday -> week 1 of 2025
+    expect(call('ISOWEEKNUM', d(2026, 9, 2))).toBe(36);
+  });
+  it('WEEKNUM is ISO by default, US-style with type 1/2', () => {
+    expect(call('WEEKNUM', d(2021, 1, 3))).toBe(53);
+    expect(call('WEEKNUM', d(2026, 1, 3), 1)).toBe(1); // Saturday, same week as 1 Jan
+    expect(call('WEEKNUM', d(2026, 1, 4), 1)).toBe(2); // Sunday starts week 2
+    expect(call('WEEKNUM', d(2026, 1, 4), 2)).toBe(1); // ... but not when weeks start Monday
+    expect(call('WEEKNUM', d(2026, 1, 5), 2)).toBe(2);
+  });
+});
+
+describe('month arithmetic', () => {
+  it('EDATE clamps to the end of a shorter month', () => {
+    expect(call('EDATE', d(2026, 1, 31), 1)).toBe(d(2026, 2, 28));
+    expect(call('EDATE', d(2024, 1, 31), 1)).toBe(d(2024, 2, 29));
+    expect(call('EDATE', d(2026, 3, 31), -1)).toBe(d(2026, 2, 28));
+    expect(call('EDATE', d(2026, 9, 2), 12)).toBe(d(2027, 9, 2));
+  });
+  it('EOMONTH', () => {
+    expect(call('EOMONTH', d(2026, 9, 2), 0)).toBe(d(2026, 9, 30));
+    expect(call('EOMONTH', d(2026, 9, 2), -1)).toBe(d(2026, 8, 31));
+    expect(call('EOMONTH', d(2026, 1, 15), 1)).toBe(d(2026, 2, 28));
+  });
+});
+
+describe('differences', () => {
+  it('DAYS counts whole calendar days, ignoring the time', () => {
+    expect(call('DAYS', d(2026, 12, 31), d(2026, 1, 1))).toBe(364);
+    expect(call('DAYS', d(2026, 1, 2) + 0.9, d(2026, 1, 1) + 0.1)).toBe(1);
+  });
+  it('DATEDIF in every unit', () => {
+    const s = d(2020, 2, 29);
+    const e = d(2026, 9, 2);
+    expect(call('DATEDIF', s, e, 'Y')).toBe(6);
+    expect(call('DATEDIF', s, e, 'M')).toBe(78);
+    expect(call('DATEDIF', s, e, 'D')).toBe(e - s);
+    expect(call('DATEDIF', s, e, 'YM')).toBe(6);
+    expect(call('DATEDIF', s, e, 'MD')).toBe(4); // 29.08. -> 02.09.
+    expect(call('DATEDIF', s, e, 'yd')).toBe(d(2026, 9, 2) - d(2026, 2, 28));
+    expect(isError(call('DATEDIF', e, s, 'D'))).toBe(true);
+    expect(isError(call('DATEDIF', s, e, 'W'))).toBe(true);
+  });
+});
+
+describe('working days (Monday-Friday)', () => {
+  it('NETWORKDAYS counts both ends and skips holidays on weekdays', () => {
+    expect(call('NETWORKDAYS', d(2026, 9, 1), d(2026, 9, 30))).toBe(22);
+    expect(call('NETWORKDAYS', d(2026, 9, 1), d(2026, 9, 30), d(2026, 9, 15))).toBe(21);
+    expect(call('NETWORKDAYS', d(2026, 9, 1), d(2026, 9, 30), d(2026, 9, 5))).toBe(22); // Saturday
+    expect(call('NETWORKDAYS', d(2026, 9, 30), d(2026, 9, 1))).toBe(-22);
+  });
+  it('WORKDAY steps over weekends and holidays, both directions', () => {
+    expect(call('WORKDAY', d(2026, 9, 4), 1)).toBe(d(2026, 9, 7)); // Fri -> Mon
+    expect(call('WORKDAY', d(2026, 9, 7), -1)).toBe(d(2026, 9, 4));
+    expect(call('WORKDAY', d(2026, 9, 4), 1, d(2026, 9, 7))).toBe(d(2026, 9, 8));
+    expect(call('WORKDAY', d(2026, 9, 2), 0)).toBe(d(2026, 9, 2));
+  });
+});
+
+describe('DATEVALUE / TIMEVALUE', () => {
+  it('read the same formats a cell entry is recognized in', () => {
+    expect(call('DATEVALUE', '2026-09-02')).toBe(d(2026, 9, 2));
+    expect(call('DATEVALUE', '02.09.2026 14:30')).toBe(d(2026, 9, 2));
+    expect(call('TIMEVALUE', '14:30')).toBeCloseTo(timeSerial(14, 30, 0), 12);
+    expect(call('TIMEVALUE', '2026-09-02 14:30')).toBeCloseTo(timeSerial(14, 30, 0), 12);
+  });
+  it('pass a number through and reject other text', () => {
+    expect(call('DATEVALUE', d(2026, 9, 2) + 0.5)).toBe(d(2026, 9, 2));
+    expect(isError(call('DATEVALUE', 'hallo'))).toBe(true);
+  });
+});
diff --git a/src/engine/__tests__/dates.spec.ts b/src/engine/__tests__/dates.spec.ts
index efecd6f..82dedf6 100644
--- a/src/engine/__tests__/dates.spec.ts
+++ b/src/engine/__tests__/dates.spec.ts
@@ -107,9 +107,16 @@ describe('parseDateTimeLiteral', () => {
     expect(parseDateTimeLiteral('2026-13-01')).toBeNull(); // month 13
   });
 
-  it('rejects an out-of-range time', () => {
-    expect(parseDateTimeLiteral('25:00')).toBeNull();
+  it('rejects an out-of-range minute/second', () => {
     expect(parseDateTimeLiteral('12:60')).toBeNull();
+    expect(parseDateTimeLiteral('37:30:60')).toBeNull();
+  });
+
+  it('reads 24 hours or more as a duration, not a time of day', () => {
+    expect(parseDateTimeLiteral('25:00')).toEqual({ kind: 'duration', serial: 25 / 24 });
+    expect(parseDateTimeLiteral('37:30')).toEqual({ kind: 'duration', serial: 37.5 / 24 });
+    expect(parseDateTimeLiteral('100:00:30')!.serial).toBeCloseTo((100 * 3600 + 30) / 86400, 12);
+    expect(parseDateTimeLiteral('23:59')!.kind).toBe('time');
   });
 
   it('does not misfire on plain numbers or arbitrary text', () => {
diff --git a/src/engine/dates.ts b/src/engine/dates.ts
index 7b677d1..0f11b18 100644
--- a/src/engine/dates.ts
+++ b/src/engine/dates.ts
@@ -38,6 +38,61 @@ export function serialToUtcDate(serial: number): Date {
   return new Date(EPOCH_MS + Math.round(serial * MS_PER_DAY));
 }
 
+/** A serial's calendar and clock fields. The time is rounded to the whole
+ *  second first, so floating-point noise (`TIME(14;30;0)` summed from parts)
+ *  never turns 14:30:00 into 14:29:59. `day` is the integer serial of the
+ *  calendar day; `weekday` follows ISO 8601 (Monday = 1 ... Sunday = 7). */
+export interface SerialParts {
+  day: number;
+  year: number;
+  month: number;
+  date: number;
+  weekday: number;
+  hour: number;
+  minute: number;
+  second: number;
+}
+export function serialParts(serial: number): SerialParts {
+  const totalSec = Math.round(serial * 86400);
+  const day = Math.floor(totalSec / 86400);
+  const sec = totalSec - day * 86400;
+  const d = new Date(EPOCH_MS + day * MS_PER_DAY);
+  return {
+    day,
+    year: d.getUTCFullYear(),
+    month: d.getUTCMonth() + 1,
+    date: d.getUTCDate(),
+    weekday: ((d.getUTCDay() + 6) % 7) + 1,
+    hour: Math.floor(sec / 3600),
+    minute: Math.floor((sec % 3600) / 60),
+    second: sec % 60,
+  };
+}
+
+/** ISO 8601 week number (weeks start Monday; week 1 is the one containing
+ *  the year's first Thursday) of a serial's calendar day. */
+export function isoWeekNumber(serial: number): number {
+  const { day, weekday } = serialParts(serial);
+  const thursday = day - weekday + 4; // the Thursday of this ISO week decides its year
+  const { year } = serialParts(thursday);
+  const jan1 = Math.round(dateToSerial(year, 1, 1));
+  return Math.floor((thursday - jan1) / 7) + 1;
+}
+
+/** Last calendar day of the month `months` after the serial's month. */
+export function endOfMonthSerial(serial: number, months: number): number {
+  const { year, month } = serialParts(serial);
+  return Math.round(dateToSerial(year, month + months + 1, 0));
+}
+
+/** Same day `months` later, clamped to that month's last day (31.01. + 1
+ *  month = 28./29.02.), like Excel's EDATE. */
+export function addMonthsSerial(serial: number, months: number): number {
+  const { year, month, date } = serialParts(serial);
+  const last = serialParts(endOfMonthSerial(serial, months)).date;
+  return Math.round(dateToSerial(year, month + months, Math.min(date, last)));
+}
+
 /** Today's *local* calendar date (a date is a calendar concept, not a
  *  timezone-relative instant), as an integer serial (no time-of-day part). */
 export function todaySerial(): number {
@@ -64,13 +119,16 @@ export function nowSerial(): number {
   );
 }
 
-export type DateLiteralKind = 'date' | 'time' | 'datetime';
+/** `'duration'` is a time span typed with 24 or more hours (`37:30`) --
+ *  shown as elapsed hours instead of wrapping to a time of day. */
+export type DateLiteralKind = 'date' | 'time' | 'datetime' | 'duration';
 
 const ISO_DATETIME_RE = /^(\d{4})-(\d{2})-(\d{2})[T ](\d{1,2}):(\d{2})(?::(\d{2}))?$/;
 const ISO_DATE_RE = /^(\d{4})-(\d{2})-(\d{2})$/;
 const DOT_DATETIME_RE = /^(\d{1,2})\.(\d{1,2})\.(\d{2}|\d{4})[T ](\d{1,2}):(\d{2})(?::(\d{2}))?$/;
 const DOT_DATE_RE = /^(\d{1,2})\.(\d{1,2})\.(\d{2}|\d{4})$/;
 const TIME_RE = /^(\d{1,2}):(\d{2})(?::(\d{2}))?$/;
+const DURATION_RE = /^(\d{1,6}):(\d{2})(?::(\d{2}))?$/;
 
 /** 2-digit year -> 4-digit, the usual spreadsheet convention: 00-29 -> 20xx,
  *  30-99 -> 19xx. */
@@ -104,7 +162,8 @@ function strictDateSerial(y: number, mo: number, d: number, h = 0, mi = 0, s = 0
  *
  * Deliberately locale-independent and conservative: only ISO dates
  * (`2026-09-02`) and dot-separated day.month.year dates (`02.09.2026`,
- * `02.09.26`) are recognized, plus `HH:MM[:SS]` time and the two combined
+ * `02.09.26`) are recognized, plus `HH:MM[:SS]` time (24+ hours: a
+ * duration) and date + time combined
  * with a space or `T`. Slash-separated dates (`02/09/2026`) are
  * intentionally NOT recognized: day-first vs. month-first is ambiguous
  * across this app's supported locales (en-US is month-first, every other
@@ -141,8 +200,17 @@ export function parseDateTimeLiteral(
     const h = +m[1];
     const mi = +m[2];
     const s = m[3] ? +m[3] : 0;
-    if (h > 23 || mi > 59 || s > 59) return null;
-    return { kind: 'time', serial: timeSerial(h, mi, s) };
+    if (mi > 59 || s > 59) return null;
+    if (h <= 23) return { kind: 'time', serial: timeSerial(h, mi, s) };
+  }
+  // 24 hours or more can't be a time of day -- it's an elapsed time
+  m = DURATION_RE.exec(t);
+  if (m) {
+    const h = +m[1];
+    const mi = +m[2];
+    const s = m[3] ? +m[3] : 0;
+    if (mi > 59 || s > 59) return null;
+    return { kind: 'duration', serial: (h * 3600 + mi * 60 + s) / 86400 };
   }
   return null;
 }
diff --git a/src/engine/functions.ts b/src/engine/functions.ts
index b660d82..e4f0b7c 100644
--- a/src/engine/functions.ts
+++ b/src/engine/functions.ts
@@ -14,7 +14,19 @@
  */
 
 import { FormulaError, isError, type CellValue } from './types';
-import { dateToSerial, timeSerial, todaySerial, nowSerial, strictNum } from './dates';
+import {
+  dateToSerial,
+  timeSerial,
+  todaySerial,
+  nowSerial,
+  strictNum,
+  serialParts,
+  isoWeekNumber,
+  endOfMonthSerial,
+  addMonthsSerial,
+  parseDateTimeLiteral,
+  type SerialParts,
+} from './dates';
 
 export type FunctionFn = (values: CellValue[]) => CellValue;
 
@@ -38,6 +50,63 @@ export function isTruthy(v: CellValue): boolean {
   return typeof v === 'number' ? v !== 0 : v !== null && v !== '';
 }
 
+/** Check the argument count and coerce every argument to a number (see
+ *  `strictNum`) -- the shared prologue of the numeric date functions. An
+ *  optional argument left out (or an empty `;;` slot) comes back as
+ *  `undefined`. */
+function numArgs(
+  name: string,
+  values: CellValue[],
+  min: number,
+  max: number,
+): [number, ...(number | undefined)[]] | FormulaError {
+  if (values.length < min || values.length > max) {
+    const range = min === max ? `${min}` : `${min}–${max}`;
+    return new FormulaError('#ERR', `${name} erwartet ${range} Argument${max === 1 ? '' : 'e'}`);
+  }
+  const out: Array<number | undefined> = [];
+  for (let i = 0; i < values.length; i++) {
+    if (i >= min && values[i] === null) {
+      out.push(undefined);
+      continue;
+    }
+    const n = strictNum(values[i]);
+    if (isError(n)) return n;
+    out.push(n);
+  }
+  return out as [number, ...(number | undefined)[]];
+}
+
+/** YEAR/MONTH/DAY/HOUR/MINUTE/SECOND: one serial in, one field out. */
+function part(name: string, pick: (p: SerialParts) => number): FunctionFn {
+  return (values) => {
+    const a = numArgs(name, values, 1, 1);
+    if (isError(a)) return a;
+    return pick(serialParts(a[0]));
+  };
+}
+
+function holidaySet(values: CellValue[]): Set<number> {
+  const out = new Set<number>();
+  for (const v of values) if (typeof v === 'number') out.add(serialParts(v).day);
+  return out;
+}
+
+function isWorkday(day: number, holidays: Set<number>): boolean {
+  return serialParts(day).weekday <= 5 && !holidays.has(day);
+}
+
+/** DATEVALUE/TIMEVALUE's single argument: a recognizable date/time text, or
+ *  a number (already a serial). */
+function textDateArg(name: string, values: CellValue[]): number | FormulaError {
+  if (values.length !== 1) return new FormulaError('#ERR', `${name} erwartet 1 Argument`);
+  const v = values[0];
+  if (typeof v === 'number') return v;
+  const hit = typeof v === 'string' ? parseDateTimeLiteral(v) : null;
+  if (!hit) return new FormulaError('#VALUE!', `${name}: '${v ?? ''}' ist kein Datum/keine Uhrzeit`);
+  return hit.serial;
+}
+
 export const FUNCTIONS: Record<string, FunctionFn> = {
   SUM(values) {
     return nums(values).reduce((a, b) => a + b, 0);
@@ -157,6 +226,171 @@ export const FUNCTIONS: Record<string, FunctionFn> = {
     return nowSerial();
   },
 
+  // ---- date/time parts & calendar arithmetic ----------------------------
+  // All take date serials (see dates.ts). Where Excel has a US-centric
+  // default, the ISO 8601 convention is the default here: weeks start on
+  // Monday (WEEKDAY -> 1 = Monday), week numbers are ISO weeks.
+  YEAR: part('YEAR', (p) => p.year),
+  MONTH: part('MONTH', (p) => p.month),
+  DAY: part('DAY', (p) => p.date),
+  HOUR: part('HOUR', (p) => p.hour),
+  MINUTE: part('MINUTE', (p) => p.minute),
+  SECOND: part('SECOND', (p) => p.second),
+
+  // WEEKDAY(Datum; [Typ]) -- Typ as in Excel: 1 = Sunday 1 ... Saturday 7,
+  // 2 = Monday 1 ... Sunday 7 (ISO, the default here), 3 = Monday 0 ...
+  // Sunday 6.
+  WEEKDAY(values) {
+    const a = numArgs('WEEKDAY', values, 1, 2);
+    if (isError(a)) return a;
+    const iso = serialParts(a[0]).weekday;
+    switch (a[1] ?? 2) {
+      case 1:
+        return (iso % 7) + 1;
+      case 2:
+        return iso;
+      case 3:
+        return iso - 1;
+      default:
+        return new FormulaError('#VALUE!', 'WEEKDAY: Typ muss 1, 2 oder 3 sein');
+    }
+  },
+
+  ISOWEEKNUM(values) {
+    const a = numArgs('ISOWEEKNUM', values, 1, 1);
+    if (isError(a)) return a;
+    return isoWeekNumber(a[0]);
+  },
+
+  // WEEKNUM(Datum; [Typ]) -- ISO week by default (Typ 21, as in Excel);
+  // Typ 1 = weeks start Sunday, 2 = weeks start Monday, both counting the
+  // week containing 1 January as week 1 (Excel's US-style numbering).
+  WEEKNUM(values) {
+    const a = numArgs('WEEKNUM', values, 1, 2);
+    if (isError(a)) return a;
+    const type = a[1] ?? 21;
+    if (type === 21) return isoWeekNumber(a[0]);
+    if (type !== 1 && type !== 2) return new FormulaError('#VALUE!', 'WEEKNUM: Typ muss 1, 2 oder 21 sein');
+    const p = serialParts(a[0]);
+    const jan1 = Math.round(dateToSerial(p.year, 1, 1));
+    const jan1Iso = serialParts(jan1).weekday; // 1 = Monday
+    const offset = type === 2 ? jan1Iso - 1 : jan1Iso % 7; // days since week start
+    return Math.floor((p.day - jan1 + offset) / 7) + 1;
+  },
+
+  EDATE(values) {
+    const a = numArgs('EDATE', values, 2, 2);
+    if (isError(a)) return a;
+    return addMonthsSerial(a[0], Math.trunc(a[1] ?? 0));
+  },
+
+  EOMONTH(values) {
+    const a = numArgs('EOMONTH', values, 2, 2);
+    if (isError(a)) return a;
+    return endOfMonthSerial(a[0], Math.trunc(a[1] ?? 0));
+  },
+
+  // DAYS(Ende; Start) -- whole calendar days between two dates.
+  DAYS(values) {
+    const a = numArgs('DAYS', values, 2, 2);
+    if (isError(a)) return a;
+    return serialParts(a[0]).day - serialParts(a[1] ?? 0).day;
+  },
+
+  // DATEDIF(Start; Ende; Einheit) -- "Y", "M", "D" complete years/months/
+  // days, "MD"/"YM"/"YD" the remainder ignoring months+years/years.
+  DATEDIF(values) {
+    if (values.length !== 3) {
+      return new FormulaError('#ERR', 'DATEDIF erwartet 3 Argumente (Start; Ende; Einheit)');
+    }
+    const s0 = strictNum(values[0]);
+    if (isError(s0)) return s0;
+    const e0 = strictNum(values[1]);
+    if (isError(e0)) return e0;
+    const unit = typeof values[2] === 'string' ? values[2].trim().toUpperCase() : '';
+    const s = serialParts(s0);
+    const e = serialParts(e0);
+    if (s.day > e.day) return new FormulaError('#VALUE!', 'DATEDIF: Start liegt nach Ende');
+    let months = (e.year - s.year) * 12 + (e.month - s.month);
+    if (e.date < s.date) months -= 1;
+    switch (unit) {
+      case 'D':
+        return e.day - s.day;
+      case 'M':
+        return months;
+      case 'Y':
+        return Math.floor(months / 12);
+      case 'YM':
+        return months % 12;
+      case 'MD':
+        return e.day - addMonthsSerial(s.day, months);
+      case 'YD': {
+        const years = Math.floor(months / 12);
+        return e.day - addMonthsSerial(s.day, years * 12);
+      }
+      default:
+        return new FormulaError('#VALUE!', 'DATEDIF: Einheit muss "Y", "M", "D", "MD", "YM" oder "YD" sein');
+    }
+  },
+
+  // NETWORKDAYS(Start; Ende; [Feiertage…]) -- Monday-Friday days in the
+  // span, both ends included, minus any listed holiday on a weekday.
+  NETWORKDAYS(values) {
+    if (values.length < 2) {
+      return new FormulaError('#ERR', 'NETWORKDAYS erwartet mindestens 2 Argumente (Start; Ende; [Feiertage])');
+    }
+    const s0 = strictNum(values[0]);
+    if (isError(s0)) return s0;
+    const e0 = strictNum(values[1]);
+    if (isError(e0)) return e0;
+    const holidays = holidaySet(values.slice(2));
+    const a = serialParts(s0).day;
+    const b = serialParts(e0).day;
+    const [lo, hi] = a <= b ? [a, b] : [b, a];
+    if (hi - lo > 366 * 1000) return new FormulaError('#VALUE!', 'NETWORKDAYS: Zeitraum zu groß');
+    let n = 0;
+    for (let d = lo; d <= hi; d++) if (isWorkday(d, holidays)) n++;
+    return a <= b ? n : -n;
+  },
+
+  // WORKDAY(Start; Tage; [Feiertage…]) -- the date `Tage` working days
+  // (Monday-Friday, not a holiday) after (or, negative, before) Start.
+  WORKDAY(values) {
+    if (values.length < 2) {
+      return new FormulaError('#ERR', 'WORKDAY erwartet mindestens 2 Argumente (Start; Tage; [Feiertage])');
+    }
+    const s0 = strictNum(values[0]);
+    if (isError(s0)) return s0;
+    const n0 = strictNum(values[1]);
+    if (isError(n0)) return n0;
+    let left = Math.trunc(Math.abs(n0));
+    if (left > 366 * 1000) return new FormulaError('#VALUE!', 'WORKDAY: zu viele Tage');
+    const step = n0 < 0 ? -1 : 1;
+    const holidays = holidaySet(values.slice(2));
+    let d = serialParts(s0).day;
+    while (left > 0) {
+      d += step;
+      if (isWorkday(d, holidays)) left--;
+    }
+    return d;
+  },
+
+  // DATEVALUE("2026-09-02") / TIMEVALUE("14:30") -- the date resp. time part
+  // of a text in any format a cell entry is recognized in (ISO, 02.09.2026,
+  // 14:30, …). A number (a cell already holding a date) passes through.
+  DATEVALUE(values) {
+    const v = textDateArg('DATEVALUE', values);
+    if (isError(v)) return v;
+    return serialParts(v).day;
+  },
+
+  TIMEVALUE(values) {
+    const v = textDateArg('TIMEVALUE', values);
+    if (isError(v)) return v;
+    const p = serialParts(v);
+    return (p.hour * 3600 + p.minute * 60 + p.second) / 86400;
+  },
+
   // XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; [WennNichtGefunden];
   // [Vergleichsmodus]; [Suchmodus]) -- like IF, evaluator.ts special-cases
   // this name *before* ever reaching FUNCTIONS. Two reasons, not just one:
@@ -206,6 +440,23 @@ export const BUILTIN_SIGNATURES: ReadonlyArray<{ name: string; hint: string }> =
   { name: 'TIME', hint: 'TIME(Std; Min; Sek)' },
   { name: 'TODAY', hint: 'TODAY()' },
   { name: 'NOW', hint: 'NOW()' },
+  { name: 'YEAR', hint: 'YEAR(Datum)' },
+  { name: 'MONTH', hint: 'MONTH(Datum)' },
+  { name: 'DAY', hint: 'DAY(Datum)' },
+  { name: 'HOUR', hint: 'HOUR(Zeit)' },
+  { name: 'MINUTE', hint: 'MINUTE(Zeit)' },
+  { name: 'SECOND', hint: 'SECOND(Zeit)' },
+  { name: 'WEEKDAY', hint: 'WEEKDAY(Datum; Typ?)' },
+  { name: 'ISOWEEKNUM', hint: 'ISOWEEKNUM(Datum)' },
+  { name: 'WEEKNUM', hint: 'WEEKNUM(Datum; Typ?)' },
+  { name: 'EDATE', hint: 'EDATE(Datum; Monate)' },
+  { name: 'EOMONTH', hint: 'EOMONTH(Datum; Monate)' },
+  { name: 'DAYS', hint: 'DAYS(Ende; Start)' },
+  { name: 'DATEDIF', hint: 'DATEDIF(Start; Ende; Einheit)' },
+  { name: 'NETWORKDAYS', hint: 'NETWORKDAYS(Start; Ende; Feiertage?)' },
+  { name: 'WORKDAY', hint: 'WORKDAY(Start; Tage; Feiertage?)' },
+  { name: 'DATEVALUE', hint: 'DATEVALUE(Text)' },
+  { name: 'TIMEVALUE', hint: 'TIMEVALUE(Text)' },
   { name: 'XLOOKUP', hint: 'XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; …)' },
 ];
 
diff --git a/src/store/__tests__/document.spec.ts b/src/store/__tests__/document.spec.ts
index fbc1e81..3ddfe3d 100644
--- a/src/store/__tests__/document.spec.ts
+++ b/src/store/__tests__/document.spec.ts
@@ -133,6 +133,35 @@ describe('seed document acceptance (canvas model)', () => {
     expect(s.canUndo.value).toBe(false);
   });
 
+  it('a formula over times shows as a duration, past 24 hours', () => {
+    const s = fresh();
+    s.newDocument();
+    const t = tableId(s, 'Tabelle 1');
+    setCell(s, 'Tabelle 1', 'A1', '08:30');
+    setCell(s, 'Tabelle 1', 'A2', '09:15');
+    setCell(s, 'Tabelle 1', 'A3', '20:00');
+    setCell(s, 'Tabelle 1', 'A4', '=SUM(A1:A3)');
+    setCell(s, 'Tabelle 1', 'B1', '=A2-A1');
+    setCell(s, 'Tabelle 1', 'B2', '=A4+B1'); // chain: a sum of formulas
+    expect(s.cellDisplay(t, 0, 3)).toBe('37:45');
+    expect(s.cellDisplay(t, 1, 0)).toBe('00:45');
+    expect(s.cellDisplay(t, 1, 1)).toBe('38:30');
+    // an explicit format always wins over the inferred one
+    s.setRangeFormat(t, { col: 0, row: 3 }, { col: 0, row: 3 }, { numFmt: 'time' });
+    expect(s.cellDisplay(t, 0, 3)).toBe('13:45');
+  });
+
+  it('a typed 24+ hour time becomes a duration; dates are not durations', () => {
+    const s = fresh();
+    s.newDocument();
+    const t = tableId(s, 'Tabelle 1');
+    setCell(s, 'Tabelle 1', 'A1', '37:30');
+    expect(s.cellDisplay(t, 0, 0)).toBe('37:30');
+    setCell(s, 'Tabelle 1', 'C1', '2026-09-02');
+    setCell(s, 'Tabelle 1', 'C2', '=C1+30');
+    expect(s.cellDisplay(t, 2, 1)).not.toContain(':');
+  });
+
   it('inserting a new table and referencing it across sheets works', () => {
     const s = fresh();
     const sheet2 = s.doc.sheets[1].id;
diff --git a/src/store/__tests__/format.spec.ts b/src/store/__tests__/format.spec.ts
index 4748b95..ebae8ab 100644
--- a/src/store/__tests__/format.spec.ts
+++ b/src/store/__tests__/format.spec.ts
@@ -23,6 +23,18 @@ describe('formatValue: date/time', () => {
     expect(formatValue(dayAtNoon, 'plain', 'datetime')).toBe('2026-09-02 12:00');
   });
 
+  it('date + time shows only the time for a bare time of day (no 1899 date)', () => {
+    expect(formatValue(timeSerial(12, 23, 0), 'de-DE', 'datetime')).toBe('12:23');
+  });
+
+  it('"duration" keeps counting hours past 24', () => {
+    expect(formatValue(37.5 / 24, 'de-DE', 'duration')).toBe('37:30');
+    expect(formatValue(timeSerial(8, 5, 0), 'de-DE', 'duration')).toBe('08:05');
+    expect(formatValue((100 * 3600 + 30) / 86400, 'de-DE', 'duration')).toBe('100:00:30');
+    expect(formatValue(-1.5 / 24, 'de-DE', 'duration')).toBe('-01:30');
+    expect(formatValue(0, 'de-DE', 'duration')).toBe('00:00');
+  });
+
   it('empty cell / error still short-circuit before any date formatting', () => {
     expect(formatValue(null, 'de-DE', 'date')).toBe('');
   });
diff --git a/src/store/document.ts b/src/store/document.ts
index 6371b61..37aa7bd 100644
--- a/src/store/document.ts
+++ b/src/store/document.ts
@@ -62,6 +62,7 @@ import type {
   ButtonObject,
   CellCoord,
   CellFormat,
+  CellNumFmt,
   CellGraphNode,
   ChartObject,
   CustomFunction,
@@ -1767,11 +1768,31 @@ function createStore() {
     return wb.cellValue(tableId, col, row);
   }
   function cellDisplay(tableId: string, col: number, row: number): string {
-    return formatValue(
-      cellValue(tableId, col, row),
-      effectiveLocale(tableId),
-      tableById(tableId)?.formats?.[`${col},${row}`]?.numFmt ?? 'auto',
-    );
+    return formatValue(cellValue(tableId, col, row), effectiveLocale(tableId), displayNumFmt(tableId, col, row));
+  }
+  /** The cell's number format, or -- for a formula cell left on "auto" -- a
+   *  format inferred from what it computes with: a formula whose numeric
+   *  inputs are all times/durations (`=SUM(B2:B8)`, `=C2-B2` over clock
+   *  times) shows as a duration, so a total of 37.5 hours reads `37:30`
+   *  instead of wrapping to `13:30` or showing the raw day fraction. Walks
+   *  formula chains (a sum of sums), bounded by `budget` cells visited. */
+  function displayNumFmt(
+    tableId: string,
+    col: number,
+    row: number,
+    budget = { left: 2000 },
+  ): CellNumFmt {
+    const explicit = tableById(tableId)?.formats?.[`${col},${row}`]?.numFmt ?? 'auto';
+    if (explicit !== 'auto' || !cellInput(tableId, col, row).startsWith('=')) return explicit;
+    let sawTime = false;
+    for (const p of wb.graphPrecedents(tableId, col, row)) {
+      if (--budget.left < 0) return 'auto';
+      if (typeof wb.cellValue(p.tableId, p.col, p.row) !== 'number') continue;
+      const f = displayNumFmt(p.tableId, p.col, p.row, budget);
+      if (f !== 'time' && f !== 'duration') return 'auto';
+      sawTime = true;
+    }
+    return sawTime ? 'duration' : 'auto';
   }
 
   // ---- number-format locale (object -> sheet -> document cascade) ---
diff --git a/src/store/format.ts b/src/store/format.ts
index 67cb3a5..7b82df4 100644
--- a/src/store/format.ts
+++ b/src/store/format.ts
@@ -46,6 +46,7 @@ export const NUM_FORMATS: ReadonlyArray<{ id: CellNumFmt; label: string }> = [
   { id: 'dateiso', label: 'Datum (ISO) · 2026-09-02' },
   { id: 'time', label: 'Zeit · 14:30' },
   { id: 'datetime', label: 'Datum + Zeit · 02.09.2026 14:30' },
+  { id: 'duration', label: 'Dauer · 37:30' },
 ];
 const NUM_FMT_IDS = new Set(NUM_FORMATS.map((f) => f.id));
 export function isCellNumFmt(x: unknown): x is CellNumFmt {
@@ -92,6 +93,18 @@ function formatTimePart(d: Date, locale: NumberLocale): string {
   }).format(d);
 }
 
+/** Elapsed time as `[h]:mm` -- hours keep counting past 24 (`37:30`), the
+ *  way a sum of working times should read; seconds only when there are any.
+ *  Locale-independent: `:` is the time separator in every supported locale. */
+function formatDuration(n: number): string {
+  let sec = Math.round(Math.abs(n) * 86400);
+  const h = Math.floor(sec / 3600);
+  sec -= h * 3600;
+  const m = Math.floor(sec / 60);
+  const s = sec - m * 60;
+  return `${n < 0 && (h || m || s) ? '-' : ''}${pad2(h)}:${pad2(m)}${s ? `:${pad2(s)}` : ''}`;
+}
+
 /** Render a computed value for display (grid cell / status bar / chart). */
 export function formatValue(
   v: CellValue,
@@ -128,7 +141,12 @@ export function formatValue(
     case 'time':
       return formatTimePart(serialToUtcDate(n), locale);
     case 'datetime':
+      // a bare time of day (< 1 day) has no calendar date -- showing the
+      // epoch "30.12.1899" in front of it would be noise
+      if (n >= 0 && n < 1) return formatTimePart(serialToUtcDate(n), locale);
       return `${formatDatePart(serialToUtcDate(n), locale)} ${formatTimePart(serialToUtcDate(n), locale)}`;
+    case 'duration':
+      return formatDuration(n);
     default: // 'auto'
       return locale === 'plain'
         ? String(n)
diff --git a/src/store/types.ts b/src/store/types.ts
index c090d3e..3fc7fef 100644
--- a/src/store/types.ts
+++ b/src/store/types.ts
@@ -27,8 +27,9 @@ export type CellNumFmt =
   | 'pct2' // percent, 2 decimals
   | 'date' // e.g. 02.09.2026 -- value is a date serial, see engine/dates.ts
   | 'dateiso' // e.g. 2026-09-02 -- ISO 8601, same regardless of locale
-  | 'time' // e.g. 14:30
-  | 'datetime'; // date + time
+  | 'time' // e.g. 14:30 -- time of day, wraps at 24 h
+  | 'datetime' // date + time
+  | 'duration'; // e.g. 37:30 -- elapsed hours, never wraps at 24 h
 
 /** Explicit formatting for a single cell (all fields optional = inherit). */
 export interface CellFormat {