How do I add photo at the time it was taken?

You can choose how photos are added to a moment with the 'Attach Photos to this moment' switch at the top of the 'Add Photos' screen.

If this is switched ON then the photos will be added to the current moment (inheriting that moments date and time).

If this is switched OFF then the photos will be added to your journal at the time they were taken. This will in effect create multiple moments if there a multiple photos.