It isn't that simple because that's not what the OP asked for - and God isn't the only one who knows what a callback function is.
The response from @BillHo is precisely what is required. I've just used it in a program of mine, after a bit of modification to use the information from a tm_elements structure instead of using rtc.now(). It does indeed update the file's datetime stamp so that it shows in Windows explorer.
I also used SdFile::dateTimeCallbackCancel(). In my code, I create a new file at the beginning of each UTC day but I open/write/close the file every minute to log the data. If I don't cancel the callback, the date/time stamp will be updated every time I open the file. Cancelling it after the file has been created ensures that the date stamp on the file will be when it was created, not when it was last written. That's the theory anyway, I'll have to wait until after 00:00:00Z before I can check that the file's date/time is correct.
Pete