I can't give you a simple answer, but perhaps I can point you in the right direction.
What you are doing when you change the time/date on your photos is changing its metadata. Metadata is text contained within the file that is read by programs such as the Mac OS Finder when you use
Get Info, photo programs, etc... Some programs allow you to edit the metadata, such the
Image-->Adjust Date and Time... menu item within the Mac OS Photo app.
More generally, media file metadata can be changed using programs called
metadata editors. For example, exiftool is a command-line metadata editor that is available on Windows, Mac, etc... TagScanner is a WYSIWYG metadata editor on Windows.
Both of the above are quite powerful, but also have a learning curve to use them. I'm guessing you'll want a simple editor for Mac OS, but I can't recommend one since I've been happily using the above for my needs. I briefly looked into the availability of metadata editors for the iPhone and found a couple (e.g., Photo Investigator, MetaPho) that claim to do what you want, but cannot be tested for free (they cost $3-$4).
One other thing: there are usually multiple types of date-timestamps for each file, so another thing you may need to discover is which is important for your needs. For example, using the
exiftool file.mp4 I see 9 timestamps for a recent movie I was sent via WhatsApp:
File Name : file.mp4
File Modification Date/Time : 2019:09:11 10:24:45+02:00
File Access Date/Time : 2019:09:11 10:24:44+02:00
File Inode Change Date/Time : 2019:09:11 10:24:58+02:00
Create Date : 2019:08:18 13:04:29
Modify Date : 2019:08:18 13:04:30
Track Create Date : 2019:08:18 13:04:29
Track Modify Date : 2019:08:18 13:04:30
Media Create Date : 2019:08:18 13:04:29
Media Modify Date : 2019:08:18 13:04:30
(to focus on the timestamps, I've omitted most of the
exiftool metadata listing)