function! MakeStamp()
  let time = strftime("%Y-%m-%d at %H:%M:%S")
  let line = getline(1)
  redi @+
  echo time
  redi END

  let match = '\d\{4}\-\d\{2}\-\d\{2} at \d\{2}:\d\{2}:\d\{2}'

  if match(line, match) >= 0
    exe '1,1s/'.match.'/'.time.'/i'
  else
    normal ggO
    normal ggO
    normal "+p
    normal dd
  endif
  return ""
endfunction

map <F3> :call MakeStamp()<CR>