[kinda ot] writing the date into a file when saving it

[rob_spellberg]

dear sir or madam ---

this may be a vi question, but i'd like to be editor-independent, if possible.

i want to self-document source code files when i write them to disk.

this would include such things as path and modification time.

ideally, within vi, i would like to have :w run a script [ that i would write ]
   that does exactly what i want.

for years, i've been doing this more or less haphazardly during development,
   until i was satisfied that the file was in its final form.
then i would manually get it right and leave it alone.
but i'm writing too much right now to keep doing this manually and
   i'm something of a nut for documentation.

its easy enough to write a sed script to find a unique string and do a replacement.
its only slightly more involved to write a glorified version of touch
   [ which is kinda what i want, actually ].

maybe what i want is to go into vi [ or ex, or wherever ],
   find where :w is processed and cause it to look for a script to run.

i know about :so.
i know about !command.
neither are really "it".

i've been googling for about an hour and coming up almost completely empty.
maybe there's a jargon word for what i want that i don't know.

so to get to the question:  what do you folks do?

rob spellberg
woodstock, illinois