Thunderbird suddenly uses 24hr time format

Within the last few days Tbird has suddenly started using 24hr time format for everything; both message dates and the calendar rather than the desired 12hr AM/PM. I've made no changes and can't figure out how to change it back. Possibly caused by an update to either Thunderbird, Linux Mint or the Linux kernel, all of which occur pretty regularly - can't say. Only update that looks like it could possibly be causing it is tzdata update from 2021a-0ubuntu0.18.04 to 2021a-2ubuntu0.18.04 on 2021-10-06.

Thunderbird 91.2.0 ubuntu-snap-build-1.0 Linux Mint Cinnamon 19.3 Kernel 5.4.0-87 generic System locale is en_US.UTF-8