title: convert date to numeric and back to date
body: <p>I have some date that I am trying to convert them to numbers and then back to original date.</p>
  
  <pre><code>      Date
  1990-12-31 
  1991-12-31 
  1992-12-31 
  1993-12-31 
  1994-06-30 
  1994-12-31 
  </code></pre>
  
  <p>I tried, </p>
  
  <pre><code>as.numeric(DF[1:6])
  [1]  1  2  3  5  7
  as.Date(as.numeric(DF[1:6]), "1990-12-31")
  [1] "1991-01-01" "1991-01-02" "1991-01-03" "1991-01-05" "1991-01-07" "1991-01-08"
  </code></pre>
  
  <p>I notice the problem of time interval. What should I do to get original dates?</p>