Data exported from Letterboxd does not include full datetimes on items, but rather just the date. When importing data from Letterboxd, Simkl interprets that date as being 12 AM UTC on that day.

This can result in an off-by-one day error when converting the date to the user's current timezone.

For example, if a movie's watched date is 2026-07-20 in the Letterboxd export data, Simkl will record the movie as having been watched on 2026-07-20 at 12 AM UTC. If a user in Los Angeles then views their watch history in Simkl, it will show the movie as having been watched on 2026-07-19 at 5 PM.

Defaulting to the user's selected timezone rather than UTC could help address this issue. Some arbitrary time will still need to be chosen since Simkl requires a time to be associated. Even with a correct timezone, 12 AM might not be the best default since it means the off-by-one issue could reoccur if the user moves even one timezone over and wants to update their account's timezone to reflect that move. Something like 12 PM might be a bit more resilient in that the date will still be accurate when moving up to ~12 timezones over in either direction.

I imagine this issue could be present with other data sources as well, depending on whether they use full datetimes and whether they include a timezone/use UTC. I can only speak from experience for Letterboxd data, however.

(Personal take, feel free to ignore) As a side note, I'm not a huge fan of watch dates being timezoned in the first place. If I watched a movie at 8 PM in one timezone, I'm more interested in seeing it displayed as 8 PM no matter where I am. I suppose that functionality can be accomplished by just not updating the timezone setting when moving, however, so maybe it's a non-issue.