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Solution: I found this this bugzilla comment which links to this page for time formatting. I changed intl.date_time.pattern_override.time_short to h:mm a so it shows 4:26PM. There's an open bugzilla report for my issue: Date not formatted according to user's locale (LC_TIME).
For instance, 4:26PM instead of 16:26, I'm on linux (NixOS) if that helps.
The only thing I can find is that it's carried over from system settings. He was running Ubu so system settings are likely your issue.
https://support.mozilla.org/da/questions/1348801