update last-modified, but record changes only if :w

hi,

I catched this little code to update last-modified content in HTML files

: au FileType x*html
:    \ exe 'g/http-equiv="last-modified"/s/content=".*/content="'.strftime("%Y-%m-%d %T").'">'

the problem is this happens even I don't record file (:w), and so date changes even there are no changes.
this doesn't make much sense, does it? :)

how could I make update recorded only if I write to file?