Rearrange Photos Not Working

Am I missing something?

In a gallery, I keep trying Rearrange gallery > Date/time taken > Ascending > update order, and it just shuffles them into some weird order that makes no sense.
I had to keep the document name because that is how they were sold. There's close to 500 pictures in the gallery and I don't have the time to rearrange it manually.

Is it me? Is there a way to sort correctly?

Thanks

It sounds like the date/ time taken wasn't included in the file data  so it doesn't have anything to sort by. 

Click on a thumbnail in the gallery and on the right of the screen  it will show date taken if there is one where it also shows date uploaded.