title: POSIXct back to character, specific format
body: <p>lets say we have  a character vector of like: </p>
  
  <pre><code>vec &lt;- "1/1/2000"
  </code></pre>
  
  <p>at some pont we turn this into a date via</p>
  
  <pre><code>newvec &lt;- as.POSIXct("1/1/2000", format = "%d/%m/%Y")
  </code></pre>
  
  <p>Now let say we want to make it a character again we can do:</p>
  
  <pre><code>as.character(newvec)
  [1] "2000-01-01"
  </code></pre>
  
  <p>the new format is <code>Y-m-d</code>. How can we turn newvec into <code>"1/1/2000"</code> ?</p>