The only way I can get the program to use international style dates is to use a non-English language, like French or Hebrew. I don't like Hebrew because it puts the screen from right-to-left. I don't like French because the week starts on Monday (whereas locally it starts on Sunday, like the US (UPDATE: this can be changed in the preferences) . I've tried changing the operating system to various locales going into Thunderbird's about:config, downloading language packs, etc. It's a bug; there's no solution in the meantime.
For other parts of the operating system I always use the ISO 8601 date*, which avoids confusion and is the default for some countries, such as Canada, and an accepted format for many other countries including the US and the UK. But Thunderbird doesn't support it and in Thunderbird 78 there is no way to adopt a custom format.
- I guess I should be consistent, and switch the long form of the date around too, but I'm not sure there's an ISO format for that.
It's hard to beat webmail such as offered by Google and Fastmail these days, but using a client like Thunderbird still has some advantages.
Ones that I can immediately think of:
- the ability to edit the subject line of incoming mail.
- "Edit as new".
- Easier to compose email correctly, with responses interspersed with the quoted original.*
- PGP and digital signatures.
- Editing the subject line: This requires the Addon "Editemailsubject MX". There is a problem with IMAP, however, that the changes occur only in Thunderbird's database, and can revert back.