title: date from number of days in R?
body: <p>I have col of number of days since 1970-01-01 00:00:00 UTC that I want to convert to date and time UTC.</p>
  
  <p>I tried this:</p>
  
  <pre><code> z &lt;- Sys.time()
   j=floor(unclass(z)/86400) # the number of days since 1970-01-01 (UTC)
  as.POSIXct(j, origin = "1970-01-01")
  [1] "1970-01-01 05:53:22 CET"
  </code></pre>
  
  <p>But it is not correct. any idea?</p>