This also threw me for a loop, and affected a lot of sites (calendars, calendly, email, discord in browser, etc) -- I'd been living with it for months, but finally had one scheduling mistake too many and grew to care enough to even log into these forums and share what I did so someone else can hopefully suffer a little less.
In my case, primarily running i3 as a window manager, I put this in .xprofile
export TZ='America/Los_Angeles'
I don't quite understand why other applications (and some websites) seemed unaffected by having this TZ variable unset, but c'est la vie.
Personally I think using TZ as a default is reasonable, and falling back to date or hwclock is also reasonable... I'd really prefer to have a setting in Vivaldi to override any of these, though I can appreciate that coordinating time is not trivial.