293431 2025-05-22 09:18:46 -0700 Incorrect date formatting in Gregorian-derived calendars before 1582 2025-05-28 20:39:34 -0700 1 1 1 Unclassified WebKit JavaScriptCore Safari Technology Preview Unspecified Unspecified NEW InRadar P2 Normal --- 1 govett webkit-unassigned karlcow webkit-bug-importer ysuzuki oldest_to_newest 2118990 0 govett 2025-05-22 09:18:46 -0700 When formatting dates before 1582 in Gregorian-derived calendars such as buddhist and japanese, the month and day are incorrect. ``` new Date(1580, 0, 1).toLocaleDateString('th-TH-u-ca-buddhist') // => '22/12/2122' (incorrect) ``` As discussed in https://github.com/tc39/ecma402/issues/1003, all of these calendars should be defined as proleptic - meaning they extend before the Gregorian Calendar was invented in 1582. WebKit overrides ICU's default Gregorian cutover date here: https://github.com/WebKit/WebKit/blob/c35dad9edf1cbad19d9af6749d43c7329eada970/Source/JavaScriptCore/runtime/IntlDateTimeFormat.cpp#L958-L961, however this only applies to the Gregorian Calendar itself and not to its subclasses due to this code in ICU: https://github.com/unicode-org/icu/blob/b30c63d1b930610850489a67433b9c3ba55d6f43/icu4c/source/i18n/ucal.cpp#L298-L310. Seems like this should be overridden for all Gregorian-based calendars. Looks like Firefox has already made this change in version 139 beta and in that version returns the correct result: ``` new Date(1580, 0, 1).toLocaleDateString('th-TH-u-ca-buddhist') // => "1/1/2123" (correct) ``` 2119438 1 ysuzuki 2025-05-23 16:48:03 -0700 Since WebKit only uses C-ICU API, unless ucal_setGregorianChange gets fixed, we cannot do much. So this is basically ICU bug 2119439 2 webkit-bug-importer 2025-05-23 16:48:41 -0700 <rdar://problem/151952896> 2119864 3 govett 2025-05-27 10:32:26 -0700 I'm not sure whether ICU is planning on changing their behavior, but based on the code for ucal_setGregorianChange, it seems like maybe you could cast the pointer to the equivalent C++ Calendar API and use that directly?