title: pandas: force 'minute' and 'seconds' to be zero
body: <p>I have one column: line1 [ 'daytime' ]</p>
  <p>the format of this column looks like:</p>
  <pre><code>2018-02-07 17:40:29
  2018-02-07 17:41:15
  2018-02-07 17:41:55
  2018-02-07 17:42:54
  2018-02-07 17:43:44
  2018-02-07 18:02:54
  2018-02-07 18:03:44
  Name: daytime, Length: 174859, dtype: datetime64[ns]
  </code></pre>
  <p>I want to get:</p>
  <pre><code>2018-02-07 17:00:00
  2018-02-07 17:00:00
  2018-02-07 17:00:00
  2018-02-07 17:00:00
  2018-02-07 17:00:00
  2018-02-07 18:00:00
  2018-02-07 18:00:00
  </code></pre>
  <p>I want change the whole column</p>