Date Taken vs. Last Modified Time
There are photo viewers for Android that sort photos by last modified time. Is that really what you want? I’m usually interested in the date and time when a photo was taken. The last modified time is the time, when a photo file was modified the last time. If you edit a photo, e.g. by making it smaller or cropping it, the last modified time should have changed. The date/time when the picture was taken should still be the same.