title: Filter data to get only first day of the month rows
body: <p>I have a dataset of daily data. I need to get only the data of the first day of each month in the data set (The data is from 1972 to 2013). So for example I would need Index <code>20</code>, Date <code>2013-12-02</code> value of <code>0.1555</code> to be extracted.
  The problem I have is that the first day for each month is different, so I cannot use a step such as <code>relativedelta(months=1)</code>, how would I go about of extracting these values from my dataset?</p>
  <p>Is there a similar command as I have found in another post for R?</p>
  <p><a href="https://stackoverflow.com/questions/22592193/r-xts-get-the-first-dates-and-values-for-each-month-from-a-daily-time-series">R - XTS: Get the first dates and values for each month from a daily time series with missing rows</a></p>
  <pre><code>17 2013-12-05 0.1621
  18 2013-12-04 0.1698
  19 2013-12-03 0.1516
  20 2013-12-02 0.1555
  21 2013-11-29 0.1480
  22 2013-11-27 0.1487
  23 2013-11-26 0.1648
  </code></pre>