How Do I Stop Google Photos From Changing My Dates Again?
Google is not changing your dates. It never wrote them into your files. Here is how to make them permanent.
Google is not changing anything. It never wrote the dates into your files in the first place.
Google Photos keeps what it knows about a photo in its own database: the capture date, any date you corrected by hand, the location, your captions. The image file itself was left as uploaded. So every time you export, you get files with no dates inside them, and every app you open them in has to guess. It will keep happening for as long as the information lives only in Google's database.
The permanent fix is to move that information inside the photos, where it belongs. Once the date and location are written into the file, they travel with it: to a new phone, a NAS, another cloud service, or a hard drive you open in ten years. That is the point of doing this once, properly. The first 500 photos are free.