If you’re uploading via the web, iNat will assign your account’s current time zone (what @bug_girl posted) setting to all observations you upload. If I go on a trip, I try to remember to change my account’s time zone to my destination’s time zone when I upload photos from there. Doesn’t always happen, however, I often forget. Batch editing is the best way to fix it.
Here’s a discussion about setting time zones automatically: https://forum.inaturalist.org/t/automatic-time-zones/7760 Unfortunately, time zones are complicated.