title: Pandas: Find first occurrence - on daily basis in a timeseries
body: <p>I'm struggling with this so any input appreciated.  I want to iterate over the values in a dataframe column and return the first instance when a value is seen every day.  Groupby looked to be a good option for this but when using  <code>df.groupby(grouper).first()</code> with <code>grouper</code> set at daily the following output is seen.</p>
  
  <pre><code>In [95]:
  df.groupby(grouper).first()
  
  Out[95]:
              test_1
  2014-03-04     1.0
  2014-03-05     1.0
  </code></pre>
  
  <p>This is only giving the day the value was seen in test _1 and not reseting the first() on a daily basis which is what I need (see desired output below).</p>
  
  <p>I want to preserve the time this value was seen in the following format:</p>
  
  <p>This is the input dataframe:</p>
  
  <pre><code>                  test_1
  2014-03-04 09:00:00 NaN
  2014-03-04 10:00:00 NaN
  2014-03-04 11:00:00 NaN
  2014-03-04 12:00:00 NaN
  2014-03-04 13:00:00 NaN
  2014-03-04 14:00:00 1.0
  2014-03-04 15:00:00 NaN
  2014-03-04 16:00:00 1.0
  2014-03-05 09:00:00 1.0
  </code></pre>
  
  <p>This is the desired output:</p>
  
  <pre><code>                 test_1  test_output
  2014-03-04 09:00:00 NaN NaN
  2014-03-04 10:00:00 NaN NaN
  2014-03-04 11:00:00 NaN NaN
  2014-03-04 12:00:00 NaN NaN
  2014-03-04 13:00:00 NaN NaN
  2014-03-04 14:00:00 1.0 1.0
  2014-03-04 15:00:00 NaN NaN
  2014-03-04 16:00:00 1.0 NaN
  2014-03-05 09:00:00 1.0 NaN
  </code></pre>
  
  <p>I just want to mark the time when an event first occurs in a new column named test_output.</p>
  
  <p>Admins.  Please note this question is different from the other marked as a duplicate as this requires a rolling one day first occurrence.</p>