What system Google Photos use to determine the time zone for photos?

When I upload some photos, the following happens depending on the source of the picture:

• From my cell phone, it uploads the correct timezone as it is set on the phone when taking the picture (makes sense).
• When uploading some random photo from my PC, it uses the timezone from the PC setting (makes sense).
• When uploading a photo from my PC that was created by my Nikon camera, it uses the GMT+1 British Summer Time zone (why?).

The problem with the incorrect time zone of the photos from my camera is extremely frustrating, as it’s a quite complicated process to change it after upload when the album is shared and contains photos from multiple sources. Is there any way to preprocess the photos (i.e., change some metadata) so the google photos app use the correct timezone?