docs(design): plan typed dates and times

[andiwand]

🤖 Generated with Claude Code

The plan for "a typed date or time in the editor" from the open-work list of docs/design/spreadsheet-editing.md, as decisions 17 to 19. The stack that follows:

1. A date or time value states its serial, and the op carries one.
2. The ods writer writes a date and a time.
3. The xlsx writer writes them, and gives the cell a date format.
4. The sheet editor reads a typed date or time, and opens a date cell on it.

The decisions

• 17. A date is a serial counted from 1899-12-30.
  • What the files store:
    • ods stores an ISO date in office:date-value, counted from table:null-date.
    • xlsx stores a serial, in the 1900 or the 1904 system. The 1900 system counts 1900-02-29, a day that never existed.
  • CellValue gives every date and time as days since 1899-12-30, whatever the file counts from, so a host computes a date once.
  • The op is {"type": "date", "number": …, "text": …}, and "time" the same.
• 18. A written date keeps the format the cell has, or gets one.
  • ods writes office:date-value or office:time-value, with the text from the cell's date style if it has one.
  • xlsx writes the serial in the workbook's epoch, and gives a cell without a date format the built-in 14, 20, 21 or 22, as Excel does.
• 19. The editor reads a date in the locale's order.
  • ISO dates always read as dates. Otherwise the order of day, month and year and the separator come from Intl.DateTimeFormat.
  • Two-digit years follow Excel's rule: below 30 is 20xx, else 19xx. Times may have AM/PM.
  • A date cell states its serial in data-odr-value, so an unchanged commit writes nothing.

Month and day names stay English in what C++ formats, because no locale data ships. That remains an open item.