Hi
Photos gets the dates from the exif data in the file which is put there by the camera (unless these are scanned images (or other images not directly from a camera), in which case it will normally be the scan date/time or the import date/time.
It sounds like the date/time was not set correctly in the camera. If you have a lot of pictures with the same amount of incorrectness (for example - if the date was wrong by 6 months for all of them, you can select all the images, correct the date/time for one of them, and all will be adjusted by the same amount. (rather than being set to exactly the same time/date). So in this example, all the selected images would be shifted by 6 months, but the number of days/hours/minutes between each photo would stay the same.