Is it possible to update timestamps on multiple photos using new Photos app?
Select the photos you want, then select the menu command "Image" -> "Adjust Date and Time..." which pops up the window that you are looking for. It seems as though it also gives control over the time zone for images with location data.

When adjusting the date/time of a group of photos, you adjust the date for one of the images, and all the other images time stamp is changed by the same amount, enabling you to correct for time zone shifts or improperly set camera date/time issues, rather than making the time/date of all the photos identical.