title: How to return time index after reaching some threshold in a pandas data frame?
body: <p>If I have a Pandas data frame that includes time and distance traveled for example that looks like this:</p>
  
  <pre><code>2017-03-05 11:46:08 0
  2017-03-05 11:46:09 1.3
  2017-03-05 11:46:10 2.1
  2017-03-05 11:46:11 3.6
  2017-03-05 11:46:12 4.2
  2017-03-05 11:46:13 5.3
  2017-03-05 11:46:14 6.8
  2017-03-05 11:46:15 7.1
  2017-03-05 11:46:16 8.8
  2017-03-05 11:46:17 9.2
  2017-03-05 11:46:18 10.1
  2017-03-05 11:46:19 11.1
  2017-03-05 11:46:20 12.3
  2017-03-05 11:46:21 13.5
  2017-03-05 11:46:22 14.2
  2017-03-05 11:46:23 15.2
  </code></pre>
  
  <p>I want to return the time of every time we reach approximately 5 such that we return an array of time that looks like this </p>
  
  <pre><code> np.array([2017-03-05 11:46:13,  2017-03-05 11:46:18, 2017-03-05 11:46:23])
  </code></pre>