It would be nice if you could set a different date format that is not tied to the application/system locale. This could apply for the mailbox views or for replies/forward texts
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Is this what you might be looking for?
https://kb.mozillazine.org/Date_display_format#Configuring_the_date/time_system_settings_on_your_com...
I.e.:
intl.date_time.pattern_override.date_short = YYYY-MM-ddintl.date_time.pattern_override.time_short = HH:mm
never-displayed