Photos does not have the batch change command for the dates like iPhoto had. "Adjust Date&Time" is meant for correcting an incorrectly set camera clock. It will shift all dates and times of the selected photos by the same amount and not set all photos to the same
If you want to set the dates and times of scanned photos to the same fixed dates, you could use an Apple Script.
You could create yourself a script like this one: Script: Batch Change the Date and Time to a Fixed Date
When I want to use a photo as a template and batch copy all metadata (title, keywords, places, date) to other selected photos, I am using this script: Script: Lift and Stamp all Metadata from one Photo to All Other Selected Photos
I am using these scripts when I have scanned a batch of old prints or slides, that will need to be set to roughly the same date some time in the past.