You can arrange to put a time stamp in a file, so that it is updated automatically each time you edit and save the file. The time stamp must be in the first eight lines of the file, and you should insert it like this:
Time-stamp: <>
or like this:
Time-stamp: " "
Then add the function time-stamp to the hook
before-save-hook (see Hooks). When you save the file, this
function then automatically updates the time stamp with the current
date and time. You can also use the command M-x time-stamp to
update the time stamp manually. By default the time stamp is
formatted according to your locale setting (see Environment Variables) and
time zone (see Time of Day in The Emacs Lisp Reference
Manual). For customizations, see the Custom group time-stamp.