Are you viewing your photos in Photos for Mac or in photos on your iPhone?
If you are using a Mac:
You may want to check, which timezone has been applied to the photos by the Photos.app, when you imported them. You can see the timezone assigned by Photos, when you select one of the photos and use the command "Image > Adjust Date& Time". On the bottom of the "Adjust Date &Time" panel you will se the "closest city". This will tell you the Timezone Photos has assigned when you imported the photos from your camera or iPhone.
The problem ist - when you import the photos from a camera, while you are no longer in the timezone where the photos have been taken, Photos may assign different timezones on import, depending on the GPS location of the photos. If the photos does not have been tagged with GPS, Photos may assign the timezone of the system time set on your Mac and the timezones will differ for photos taken with a camera without GPS and an iPhone.
To find all photos there the time needs fixing,
- a smart album with the rule "Photo > is not > tagged with GPS" may help.
- Then select all photos in this album and assign the correct "Closest city" in the "Adjust Date & Time" dialog. This will just change the timezone name to display the date.
- Afterwards correct the "Date&Time adjusted" so it will match again the "Date&Time original."
