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/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) ---