Henri SivonenToday in Web compat: Firefox and Safari are ahead of Chrome in ICU4C version and upsteam ICU4C changed the formatting of zero offset from GMT. This broke birthday date validation for a UK based site for birthdays before 1970 in Firefox and, on 26.x Apple OSs, in Safari, because the site performs a formatting-based check on the time zone of London on the date to be validated and the UK has changed time zone rules along the way.
Dominik Röttsches@hsivonen https://chromium-review.googlesource.com/c/chromium/src/+/7623512 ICU roll landed and is coming in Chrome 148. We kept GMT instead of GMT+0 in http headers due to that being hard coded in the respective RFC, but for JS date formatting we should conform to new Unicode rules.
@drott It turned out that while the CLDR change was intentional, this particular outcome was not. https://unicode-org.atlassian.net/browse/CLDR-19362