Geotagging: time offset is not exported
Applying a time offset changes the timestamp displayed in the darktable GUI, but the adjusted timestamp is not present anywhere in the metadata of the exported (JPG) image. Exif.Image.DateTime, Exif.Photo.DateTimeOriginal and Exif.Photo.DateTimeDigitized all contain the old unadjusted value. I'd expect the offset to be applied to at least one of them (or maybe all?).
Alternatively, the GPSTimeStamp/GPSDateStamp fields might be added for the adjusted time, but these are probably not as widely supported by other software. Also, from the darktable GUI it seems this mechanism is usually meant to correct an inaccurate camera clock, so it should be no problem to overwrite the original timestamps.