Updated TB 08/Sep via the 26.1 platform o Windows 10 but TB 115.2.0 calendar now insists that event times are 2 hours later than they had originally been set: a previously set 12:00 appointment is now being displayed as a 14:00 appointment.
It makes no difference if I continue to use the system timezone or set it manually before re-starting TB.
I remain in the same GMT+0300 timezone (Eastern European Summer Time) which is 2 hours ahead of BST (I use UK settings for language, formats, etc.) and the Multiprocess Browser Toolbox shows exactly this in response to 'Date()'. The config editor has privacy.resistFingerprinting set to 'false'.
I can't guess why this has happened and haven't yet found any other mention of this elsewhere but similar was thought to be a containerisation issue many moons ago so I desperately wonder if this could this be a portabilisation issue. Clues welcome.
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You could try installing a fresh copy of Thunderbird Portable to your desktop or another location to see if it is some weirdness in your profile data.
If it is still happening on a fresh install, try running Thunderbird directly without our portability layer by running
ThunderbirdPortable/App/Thunderbird/thunderbird.exe(just note that doing this will create a profile on the local computer you may wish to clean up later).If you still see the issue running without portability it is a bug in Thunderbird itself and not something we can help with, sorry.
Thanks for your suggestions. Useful info for bypassing portability layer in future diagnoses.
To get up and running asap I used the full backup I'd created just before updating apps. This (TB 102.14.0) does not exhibit the timezone offset introduced with 102.15.0. I still have a couple of POP accounts but they're set to not auto-delete emails from server when downloaded so no incoming email lost and I avoided creating or editing events in 102.15.
I'll follow your suggestions and investigate further as soon as circumstances allow. Thanks again.