The date and time are not two distinct attributes fields or values – just one quantity of seconds since an epoch date (the opening moment of 1970), which the stringifications present in terms of a date and time combination, for readability.
What you need to do is just calculate the new moment that you require, and then restamp the file.
See, for example:
[Unix time - Wikipedia](https://en.wikipedia.org/wiki/Unix_time)
and in Terminal.app
man touch
for documentation of the touch command
and this thread for previous discussion:
Problem - How to change creation and modification date of selected files?