You can arrange to put a time stamp in a file, so that it will be updated automatically each time you edit and save the file. The time stamp has to be in the first eight lines of the file, and you should insert it like this:
or like this:
Time-stamp: " "
Then add the hook function
time-stamp to the hook
before-save-hook; that hook function will automatically update
the time stamp, inserting the current date and time when you save the
file. You can also use the command M-x time-stamp to update the
time stamp manually. For other customizations, see the Custom group
time-stamp. Note that non-numeric fields in the time stamp are
formatted according to your locale setting (see Environment).