How Do I Fix the Dates on a Very Large Photo Library?
Fifty thousand photos, several hundred gigabytes, split across many zip parts. Here is how to approach it without redoing the work.
Big libraries fail in a specific way: you get halfway, hit a problem, and have to start again. A little planning avoids that.
Unzip everything into one folder first. A photo and its .json file often sit in different parts, and matching only works when they are together. Write to a new folder rather than in place. Your original export stays untouched, so a mistake costs you a re-run rather than your library. Budget the disk space for it. Set your timezone before you start, because changing it afterwards means doing the whole run again. Test on one album first. Process a few hundred photos, check them properly, then commit to the rest.
Expect a large library to take a while. Writing metadata to a few hundred thousand files is disk-bound work, and any tool that claims it is instant is not writing the files.
The app is built for this size. It handles multi-part exports as one job, has a dry run so you can see what it will do first, and lists every file it could not match at the end rather than silently skipping them. It runs offline, so a 300 GB library is never uploaded anywhere. The first 500 photos are free, which is exactly the test run you want before starting the real one.