Mac Photos: Change Date of selected photos
The Photos app does not have a feature for changing the date of multiple photos to one particular date. You can, however, change multiple photos relative to their original date. 

Select the desired photos by either holding Shift and clicking to select multiple adjacent photos, or holding Ctrl and clicking individual photos.

In the menu bar, go to Image -> Adjust Date and Time 

You can now change the date relative to the date of each photo you selected. For example, if you selected two photos, the first being taken on Feb 13 and the second being taken on Feb 12, and then adjusted by 5 days, the photos would be changed to Feb 8 and Feb 7.