[JavaScript / Python] Make a pass to implement missing pre-defined Date format #3976

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1. MangelMaxime

   added

   enhancement

   pythonPythonPython

   javascript

   on Dec 16, 2024
2. github-actions commented on Apr 6, 2026

   

   github-actionsBot

   on Apr 6, 2026 – with GitHub Actions

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   🤖 This is an automated response from Repo Assist.

   Thanks for filing this! I investigated the current state of standard DateTime format specifiers in both the JavaScript/TypeScript and Python runtime libraries.

   Current State

   Both dateToStringWithKind (JS: src/fable-library-ts/Date.ts, Python: src/fable-library-py/fable_library/date.py) only handle a handful of single-character format specifiers in their length === 1 switch:

   Specifier	Description	JS	Python
   d	Short date	✅	✅
   D	Long date	✅	✅
   T	Long time	✅	✅
   t	Short time	✅	✅
   O / o	Round-trip (ISO 8601)	✅	✅
   R / r	RFC 1123 ("Thu, 01 Jan 2009 00:00:00 GMT")	❌	❌
   F	Full date/time (long time)	❌	❌
   f	Full date/time (short time)	❌	❌
   G	General date/time (long time)	❌	❌
   g	General date/time (short time)	❌	❌
   M / m	Month/day	❌	❌
   s	Sortable (ISO 8601 without timezone)	❌	❌
   u	Universal sortable	❌	❌
   U	Universal full	❌	❌
   Y / y	Year/month	❌	❌

   Both targets throw "Unrecognized Date print format" for all unhandled single-character specifiers. The custom-format handler (dateToStringWithCustomFormat) handles multi-character patterns fine.

   Implementation Notes

   • R/r: JS can use date.toUTCString() (already returns RFC 1123 format in modern browsers/Node.js). Python can use date.strftime("%a, %d %b %Y %H:%M:%S GMT") after converting to UTC.
   • s: ISO 8601 without timezone — JS: date.toISOString().replace('Z', '') (adjusted for local); Python: date.strftime("%Y-%m-%dT%H:%M:%S").
   • F/f, G/g, M/m, Y/y: These are locale-dependent in .NET. Since Fable doesn't have full locale support, returning the invariant culture equivalent (fixed English format) would be acceptable, similar to how D, d, T, t are implemented.

   The fix would be straightforward additions to the switch statement in both src/fable-library-ts/Date.ts and src/fable-library-py/fable_library/date.py, plus dateToStringWithOffset for completeness. Since src/fable-library-py/ is on PyPI, care should be taken not to change existing specifier semantics — only add new cases.

   Note

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3. github-actions

   added a commit that references this issue on Apr 20, 2026

   feat: add missing standard DateTime format specifiers for JS/TS and P…

   ...

   This issue will close once commit c2c7697 is merged into the 'main' branch.

   c2c7697
4. dbrattli

   closed this as completedin #4547on Apr 27, 2026