How does Photos order pictures in the "Library" view?

In the Photos app on iPhone and iPad, the "Library" view orders all the pictures/videos by date. However, for certain pictures and videos, there is unexpected behavior. For example, older pictures/videos that are synced to the device have unexpected behavior (e.g. showing up as taken at the time of sync, or the date modified of the file). Does anyone know what the Photos app uses to order by? In particular:

• What metadata tags does the app default to using?
• Is there a hierarchy for metadata tags in terms of which the app reads first?

I'm looking to update the metadata for old pictures which corresponds to the date taken, but have had mixed results for proper sorting by updating EXIF metadata tags alone.

Thanks for your thoughts!