I'm using El Capitan 10.11.1 and Photos app 1.2
The new version of the the Photos app allows you to update the metadata for photos via the 'Get Info' menu option. For a single photo, you can update the timestamp by opening the Info inspector, and double clicking the date/time.
The resulting Adjust date and time of selected photos dialog strongly implies that you should be able to do this for multiple photos at once. It says:
"The original date and time of your first photo is shown below. Adjust it to the correct time, and all 1 photos will be time-shifted accordingly."
So... I have about 100 photos I need to do this to, but if I select them all (or any number greater than 1), the Info inspector changes subtly - the timestamp changes format from e.g. "11 November 2015 12:11" to simply "11 Nov 2015", and double clicking the date now does nothing except select the date in the Info inspector.
Is it possible to shift the date/time of my 100 photos within the Photos app?