I banged my head on this several times until I came up with basically the same conclusion. PhotoStructure stores both the raw EXIF value, a nullable timezone t

I banged my head on this several times until I came up with basically the same conclusion. PhotoStructure stores both the raw EXIF value, a nullable timezone that tries to adopt sibling's timezones (normally only inferable when you have GPS tags, including GPS acquisition time, which is always in UTC). It also stores the source of the inference, so other heuristics know how much it can "trust" that value. I open-sourced the GPS location and offset inference code: https://github.com/photostructure/exiftool-vendored.js https://github.com/photostructure/exiftool-vendored.js