title: 24hour clock substraction python
body: <p>Probably a billy basic question, I can insert the current hour into my python but I want to be able to subtract 3 hours from it. I've tried the below but this just prints out the hour - 3.</p>
  
  <p>I need the time to be in 24 hour format because I need to keep the leading zero. Where am I going wrong?  </p>
  
  <pre><code>import time
  print (time.strftime("%H -3"))
  15 -3
  </code></pre>