I'm using Photos 1.5 (OSX 10.11.5) and synchronizing my library to iCloud. After allowing the 23k photos to synchronize over a couple of days, I noticed that a batch of several thousand older…
See this -- http://www.macworld.com/article/3015012/software/does-your-photo-have-the-wrong- timestamp-heres-how-to-fix-it.html