Photos has ne tool to set all selected items to the same date and time. The "Adjust date & Time" is meant to correct the a systematic time offset, if the clock in a camera has been set incorrectly and all dates and times are shifted by the same offset.
When I want to set the dates of scanned photos to same date, with as small increment between successive photos, I am arranging the photo in a top-level album in the sequence I want them to be sorted, select them all together, set the date of the first photo correctly, then run an apple script to set the date and time of the successive photos to the same date and time: Script: Batch Change the Date and Time to a Fixed Date
When I also want to copy other metadata, like the keywords and location and titles, I am using this script instead: Script: Lift and Stamp all Metadata from one Photo to All Other Selected Photos
Since the Catalina update, it is more complicated to pass selected photos to an allow script. all photos have to be selected in All Photos or in a top level album, not in nested albums in folders.
Alternately, you could use third party software like Houdah Geo to set the dates of the photos to a fixed date, before you import them to Photos for Mac.
