function! LastModified()
  if &modified
    let save_cursor = getpos(".")
    let n = min([6, line("$")])
    keepjumps execute '"1," . n . "s#^% Modified:zs.*# " . strftime("%H:%M %A, %-d %B %Y") . "#"'
    call histdel('search', -2)
    keepjumps call setpos('.', save_cursor)
  endif
endfun

autocmd BufWritePre *.tex call LastModified()

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Title:    ...some title...
% Author:   ...author...
% Modified:  8:31 Monday, 19 Mar 2018
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%