feat(ooxml): write a date and a time into an xlsx cell

[andiwand]

🤖 Generated with Claude Code

Step 3 of the typed-dates plan (decision 18, the xlsx half).

Change

• sheet_set_cell: it takes a date or time that states its number. One without a number is refused before the cell changes.

  • A date writes serial_from_days(days, epoch), so a 1904 workbook gets its own serial.
  • A time writes its length in days as it is, because a time is a duration that no epoch moves.

• The format: a cell whose format is not a date or time format gets one of ECMA-376 18.8.30, as Excel gives one when a date is typed:

  • 14 for a date;
  • 22 for a date with a time;
  • 20 for a time;
  • 21 for a time with seconds.

  A cell that already has a date format keeps it.

• StyleRegistry::create_cell_format: it takes an optional numFmtId and sets applyNumberFormat. restyle_cell passes it through.

Review after the rebase

A written time of a day or more got built-in 20, h:mm, which wraps at a day, so 42:00 showed as 18:00. It gets built-in 46, [h]:mm:ss, now, as Excel gives it, and the test writes 1.75 and reads 42:00:00. The rest holds: a time is written as it is and a date as the serial of the workbook's epoch, a cell that has a date format keeps it, and create_cell_format interns the xf with its numFmtId.

Proof

• OoxmlSpreadsheetValue.a_written_date_gets_a_date_format: a date, a date with a time, a time, and a time with seconds into plain cells. Read back after a save, they show 01-01-25, 1/1/25 18:00, 18:00 and 18:00:05, with the date and time value types.
• OoxmlSpreadsheetValue.a_date_keeps_the_date_format_its_cell_has: in a 1904 workbook, B1 keeps s="2" and gets the serial 44197.
• OoxmlSpreadsheetValue.*, OdfSheet* and NumberFormat.*: 85 passed.
• LibreOffice oracle: back_translate wrote dates and times into an xlsx: into cells with dd.mm.yyyy, hh:mm and yyyy-mm-dd hh:mm, into a plain cell, and into two new cells. LibreOffice's csv export reads every one as written: 02.01.2025,18:30,2025-01-02 18:00,01/02/2025,18:00,01/02/25 06:00 PM. The last three come from the formats 14, 20 and 22.
• g++-15 -Wall -Wextra -Werror is clean on the touched files.