title: How to remove seconds before the first full minute
body: <p>I currently have second by second data from an accelerometer that I need to average by the minute (average 60 seconds together). The issue is that I have data at the beginning and end of the first and last minute that are not a whole minute.  I want to remove those seconds so when I average by the minute, it always starts and stops on the minute.  My data is also multiple participants stacked on top of each other so I would need to do this by id.  An example of how my data is laid out is below:</p>
  
  <pre><code>ID    Timestamp
  *1     2017-03-15 10:29:58
  1     2017-03-15 10:29:59*
  1     2017-03-15 10:30:00
  1     2017-03-15 10:30:01
  ...
  *1     2017-03-15 12:48:00*
  *1     2017-03-15 12:48:01*
  *1     2017-03-15 12:48:02*
  *2     2017-04-01 11:19:59*
  2      2017-03-15 11:20:00
  </code></pre>
  
  <p>etc.</p>
  
  <p>Hopefully I can do this where I can eliminate the italicized rows for each ID so I only have whole minutes for each person. (I have about 80-95 ids per dataframe). This will allow me to aggregate by the whole minute.</p>
  
  <p>If there is an easier way to average the values on the whole minute that would not require me to remove rows, that may be easier.</p>