You would like to add an automatically updated timestamp in your text files. You can do that by adding 6 lines of code to your .vimrc.

The following command will call the function LastMod() whenever a buffer of file is written.

autocmd BufWritePre,FileWritePre *.html   call LastMod()

The following function will find all files containing ‘Last modified:’ and replace them with the current date and time.

fun LastMod()
  exe "%g/Last modified: /s/Last modified: .*/Last modified: " . strftime("%Y-%m-%d %T")
endfun

The biggest problem with this function is that using the :substitute function will move the cursor to the beginning of the line. This will even happen when nothing gets replaced. This is extremely annoying.

This can be fixed by remembering the current column and setting the cursor back to that after the command. Remembering the cursor position is easy.

let save_cursor = getpos(".")

And restoring it is not much harder.

call setpos('.', save_cursor)

I added . "/e" to the and of the regex to catch errors. Without the e flag an error will happen when there is no line matching the regex.

With all these changes the code now looks like this:

fun LastMod()
    let save_cursor = getpos(".")
    exe "%s/Last modified: .*/Last modified: " . strftime("%Y-%b-%d %X") . "/e"
    call setpos('.', save_cursor)
endfun