Update Oct 2022:
I think I fixed this in an update earlier this year and forgot to mention it. Here is the note from Feb 2022:
Fixed bug where time on one device could mis-overwrite time on another -- syncing should work properly now, give or take a few minutes.
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Thanks for the rating and feedback!

Apparently Firefox's sync feature does not do what I thought it did. Sorry, that was careless of me :/

Looks like I'll have to get rid of the sync-related code and replace it with device-specific storage. That means in the future you'd just have 12 hr on desktop and 8 hr on laptop, but no way to link the two besides in your head.