How do change date of multiple photos in photos app on mac to same exact date/time?

Unfortunately, there is not way to change all of the EXIF capture dates to the exact same date in Photos. However, there are a couple of AppleScripts created by users that can add a date to the Title field, either with or without added text and a padded sequential number. If exported with the Title as the new file name the date you assigned will be part of the file name. The AppleScripts that can do that are:

Batch Change Title to Text w/Padded Sequential Numbers

Script: Batch Change the Titles to a Given String

The first of those two will give this result:

Before:

Then add date and text:

To get this:

When exported with the title as the new file name you'll get this:

If you want to change the date outside of photos, there are a number of apps that will do that. Most are paid, however there is a very powerful tool called exif tool. As standard it is command line based, but there is also a GUI available.

https://www.macupdate.com/app/mac/52791/exiftool

Another free one that might work (limited filetype support, and not sure how up to date the development is)

https://www.qdev.de/?location=mac/exifrenamer

Or one of the paid ones:

https://www.exifedit.com/for-mac/

Take a look at some scripts written by a community member, they may help: http://www.oldtoadstutorials.net/No.P01.html.