Can we start with you have a problem and I do not. I did nothing special, just my computer (Windows) and hence my locale settings set the date format of my country. If your date is not set correctly then most likely there is a reason for this and it is not sloppy programming in Thunderbird. There is a setting in the calendar for dates. But that is not used in the user interface beyond the calendar, General dates are implemented via the system locale. On Linux where nothing can be assumed and every distribution is different as if every distribution method. The locale is set generally using two methods. One is in the GUI windowing environment and is what most Windows refugees will be relying on. There is another in the underlying system and they do not necessarily talk to one another. Personally I think it is just another reason the much talked of year of Linux on the desktop is still not here. They can not standardize on anything and there has to a customization feature to break any standard that does emerge. But I digress. You need to examine the locale setting of your system using the command line. Thunderbird uses system settings not windowing environment settings. This page appears to offer a fairly comprehensive overview https://www.tecmint.com/set-system-locales-in-linux/ For a start you appear top have forgotten to mention that this is on some Linux distribution.