title: Error in as.POSIXlt.character(x, tz, …) : character string is not in a standard unambiguous format
body: <p>I use a data set imported from excel:</p>
  
  <pre><code>OBX &lt;- read_excel("C:/Users/neri_/Desktop/OBX Weekly Return.xlsx")
  
           Date          OBX
  1   2010-12-28    0.0071366
  2   2010-12-29  4.97265e-05
  3   2010-12-30  -0.00452489
  4   2011-01-03   0.00896603
  5   2011-01-04   -0.0101488
  6   2011-01-05   0.00520143
  7   2011-01-06 -0.000422917
  8   2011-01-07  -0.00301145
  9   2011-01-10  -0.00341996
  10  2011-01-11     0.013326
  11  2011-01-12   0.00882484
  </code></pre>
  
  <p>I'm struggling with this error message, </p>
  
  <blockquote>
    <p>"Error in as.POSIXlt.character(x, tz, ...) : character string is not in a standard unambiguous format", 
    which I'm encountering when I'm trying to run the code: </p>
  </blockquote>
  
  <pre><code>weeklydata&lt;-to.weekly(OBX).
  </code></pre>
  
  <p>I want to convert daily data data to weekly data, how do I modify the date format, so I can run the to.weekly command? </p>
  
  <p>Appreciate your help!</p>