Seems like just a bug in the app or on your phone/computer... there are no settings to set the date/time it's all inherited from your devices.
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It's not an app per se. I run off the website with Librewolf.
You might be right tho cuz my phone does fine.
Well, it is still a web app ahah
Try another browser maybe, or in private browsing maybe there is an issue with your browser, never tried tuta with librewolf, but I've never had such issue with LibreWolf... maybe you have some kind of VPN or something that change your metadata/timezone ?
If it was the latter, I'd get a different time than UTC the way my BIOS runs. I Switched the VPN For a time change that did not manifest. I'm pretty sure that somehow it's librewolf but I can't find such settings over time in their settings. Or I'm practically blind. Tends to be the case when something is literally right in front of me.
Try disabling your extensions one by one to see if one of them is the culprit maybe!
I will not disable uMatrix. Ever.