Whoever used a PlayStation 5 to browse
@X:
Thanks to you, I learned that the PS5 browser doesn’t report a client timezone that conforms to the JS Intl / IANA model. It surfaces Etc/Unknown, which is not a valid timeZone for new Intl.DateTimeFormat('en', { timeZone }).format() and causes the call to fail.
Edge cases always find you eventually. This one found me from a console.