title: how to get specific date in time series in r
body: <p>I want to make a forecast project from a time series dataframe.
  but, the time span is too big.
  then, I have this column in dataframe from a time series data frame.</p>
  <pre><code>Date
  2010-06-29
  2010-06-30
  2010-07-01
  2010-07-02
  </code></pre>
  <p>how can I change it so that it only shows every 7 days?</p>
  <pre><code>Date
  2010-06-29
  2010-07-05
  2010-07-12
  2010-07-19
  etc
  </code></pre>