title: Convert tibble to xts for analysis with performanceanalytics package
body: <p>I have a tibble with a date and return column, that looks as follows:</p>
  <pre><code>&gt; head(return_series)
  # A tibble: 6 x 2
    date      return
    &lt;chr&gt;      &lt;dbl&gt;
  1 2002-01  0.0292 
  2 2002-02  0.0439 
  3 2002-03  0.0240 
  4 2002-04  0.00585
  5 2002-05 -0.0169 
  6 2002-06 -0.0686 
  </code></pre>
  <p>I first add the day to the date column with the following code:</p>
  <pre><code>return_series$date &lt;- as.Date(as.yearmon(return_series$date))
  
  # A tibble: 6 x 2
    date         return
    &lt;date&gt;        &lt;dbl&gt;
  1 2002-01-01  0.0292 
  2 2002-02-01  0.0439 
  3 2002-03-01  0.0240 
  4 2002-04-01  0.00585
  5 2002-05-01 -0.0169 
  6 2002-06-01 -0.0686 
  </code></pre>
  <p>My goal is to convert the return_series tibble to xts data to use it for further analysis with the PerformanceAnalytics package. But when I use the command as.xts I receive the following error:</p>
  <pre><code>Error in as.POSIXlt.character(x, tz, ...) : 
    character string is not in a standard unambiguous format
  </code></pre>
  <p>How can I change the format to xts or is there an other possibility to work with the PerformanceAnalytics package instead of converting to xts?</p>
  <p>Thank you very much for your help!</p>