Durations past 24 hours and the basic date/time functions

- New cell format "Dauer" ([h]:mm): hours keep counting past 24 (37:30).
  A formula left on "Automatisch" whose numeric inputs are all times or
  durations (=SUM over working times, =end-start) displays as a duration.
  Typing 24 or more hours (37:30) enters a duration.
- "Datum + Zeit" shows a bare time of day (< 1 day) without the epoch
  date 30.12.1899.
- New functions: YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, WEEKDAY,
  ISOWEEKNUM, WEEKNUM, EDATE, EOMONTH, DAYS, DATEDIF, NETWORKDAYS,
  WORKDAY, DATEVALUE, TIMEVALUE. Where Excel defaults to US conventions,
  ISO 8601 is the default: weeks start Monday (WEEKDAY -> Monday = 1),
  WEEKNUM is the ISO week. Excel's other types stay available.
- Command reference and README document all of it; the README test
  count is updated to 553.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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;