So in Mac Thunderbird 60.3.0 (which I just upgraded to), at Preferences > Advanced > General, I have my "Date and Time Formatting" setting to "Regional settings locale: English (Canada) On my Mac, I have my custom Canada locale set to the following formats: Time: 9:05 AM Date: 2018/11/08 (I'll attach screenshots of the settings.) My issue is that the date formats in Thunderbird are displaying slightly off, and I not only do I not know how to change them... I'm not totally sure where it's pulling them from. For messages from today, it's displaying just the time (great!) but with this format: 8:44 a.m. when my system should be telling it to display: 8:44 AM For messages from this week, it's displaying the day of the week and the time (also great!) but like so: Tue., 9:24 p.m. when what I'd like would be: Tue 9:24 PM And for older messages, it's displaying the short date format and the time (totally great!) but like so: 2018/10/31, 2:51 p.m. when what I'd like would be: 2018/10/31 2:51 PM Now I understand I might not be able to control the use of the comma "," as a separator between either the day of the week and the time or the short date and the time, but I don't at all understand why it's doing the following: 1) adding a period after the day of week abbreviation (i.e. it should be using "Wed" but it's using "Wed.") 2) why it's using lowercase, period-separated "a.m." and "p.m." when it should be using "AM" and "PM" Any ideas? I don't have those formats set in my regional settings, so I'm at a loss as where Thunderbird is pulling them from.