The date changes, whenever Photos is downloading an original from iCloud - when we use "Optimize Storage" or are migrating the library to a new device fusing iCloud Photos, downloading a fresh copy from iCloud, or when we are syncing the same library with several macs or other devices. If an original, that a Mac is trying to upload is already in iCloud, iCloud has to solve conflicts between identical files from several devices and will keep only one for all devices. This may change the creation date on the other devices. It is not documented, as far as I know, but what I observed on my Macs.
And I am using my iPhone more and more for taking Photos. when I am travelling, it may take several days, before I can connect to a decent internet and sync the photos from my iPhone to my mac. Most iPhone photos are showing the date as the creation date, when they have been downloaded to from iCloud to my Mac, several ays after they have been taken.