title: Add or change time in datetime object
body: <p>I have multiple datetime objects, for example:</p>
  <pre><code>&gt;&gt;a
  datetime.datetime(2009, 11, 1, 0, 0)
  </code></pre>
  <p>I want to keep date unchanged and alter time to zeros (three digits in time):</p>
  <pre><code>&gt;&gt;b
  datetime.datetime(2009, 11, 1, 0, 0, 0)
  </code></pre>
  <p>Probably simple but i can't figure it out</p>
  <p><strong>EDIT:</strong>
  For some reason a zero for seconds doesnt work. It does not matter, it works with a 1:</p>
  <pre><code>&gt;&gt;a
  datetime.datetime(2009, 11, 1, 0, 0)
  &gt;&gt;b = datetime.datetime.combine(a.date(), datetime.time(0,0,0))
  &gt;&gt;b
  datetime.datetime(2009, 11, 1, 0, 0)
  &gt;&gt;b = datetime.datetime.combine(a.date(), datetime.time(0,0,1))
  &gt;&gt;b
  datetime.datetime(2009, 11, 1, 0, 0, 1)
  </code></pre>