Bulk Changing of Year Tag?

So I have over 2000+ files that have the tag format in the year field as "1999-02-23T08:00:00Z" due to iTunes tagging it for M4A files.

I want to be able to rename it to the format of just "1999-02-23".

Is this possible to achieve in MP3TAG and have it done to bulk files in various files? I would prefer to do it in MP3TAG because I don't want to alter the date modification time of this files since I have that option ticked within the program.

This looks like the format for RELEASETIME.
So I would copy that data from YEAR to RELEASETIME and then shorten YEAR to just 4 digits - as that is the format of the YEAR field, just the year, not month or day.
To copy YEAR to RELEASETIME use an action of the type "Format value" for RELEASETIME
Format string: %year%

To get just the 4 digits into YEAR, one of the ways may be to use an action of the type "Replace with regular expression" for YEAR
Search string:^(\d+)-.*
Replace string: $1