title: Time difference between 21:00 and 06:00 from another time
body: <p>I have a website that should calculcate "extra money" for workers when they work between 21:00 and 06:00.
  How can I calculate the hours between 21:00 and 06:00? The input may be outside the range 21 and 06, eg start 19:30 and stop 07:15...</p>