I think there might still be an issue here - in particular with time zones above UTC+12. I'm currently in NZDT (UTC+13) and I've noticed that all the emails that are sent with the 'mail' command have the time zone in the 'Date' header set to -1100. I think the issue is with the 'else if (d > SECS_PER_DAY/2)' line since this will always be true for time zone offsets above UTC+12 and so it subtracts SECS_PER_DAY making my UTC+13 become UTC-11.

The introduction of message_add_date in GNU Mailutils 3.13, may lead to weirdness in the timezone offset in Date headers added when sending messages (eg "Date: Sat, 7 Aug 2021 23:00:01 +4400"). I believe the problem arises in mu_utc_offset() in libmailutils/datetime/utcoff.c when the day (or month or year) of the GMT date differs from that of the localtime date.

For example, consider a message being sent at 2005 EDT / 0005 UTC. In mu_utc_offset, TMSEC(&ltm) equals 72300 and TMSEC(&gtm) equals  300. Initially, d is assigned 72000. But since ltm.tm_mday does not equal gtm.tm_mday, the function adds a day's worth of seconds to it and returns 158400. And in turn, mu_c_streamftime() generates the timezone offset in the date header by converting that to hours (44 = 158400/(60*60)) and minutes (0 = (158400/60)%60).