Viewing Metadata
Optional but informative. Do this to see all the tags.
exiftool -a -G1 -s FILE/DIR
Modifying using a fixed date-time
exiftool "-FileModifyDate=YYYY:MM:DD HH:MM:SS" FILE/DIR
Modifying using another file type but same name in a common dir
For when you re-encode that video file.
The following copies the tags from the .mov file to the .mp4 one.
exiftool -TagsFromFile %d%f.mov "-FileModifyDate<FileModifyDate" -ext mp4 DIR
And finally using the filenames
Batch rename them as YYYYMMDDHHMMSS if not. Trust me batch renaming is easier than using a regex.
exiftool "-FileModifyDate<filename" DIR
NB: Use alldates instead of FileModifyDate first. Yes, run all twice ;)