title: R error: unknown timezone with as.POSIXct()
body: <p>I am trying to convert a unix epoch timestamps to a date-time object using as.POSIXct()</p>
  
  <p>I need to specify timezones (either Europe/London or UTC) when I call as.POSIXct().</p>
  
  <p>If I run </p>
  
  <pre><code>&gt; t&lt;-as.POSIXct(1445329330, tz="Europe/London", origin="1970-01-01")
  &gt; t
  </code></pre>
  
  <p>R returns
      "2015-10-20 09:22:10 BST"
      Warning messages:
      1: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London'
      2: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London'</p>
  
  <p>I have tried specifying tz="BST", but this also returns warnings</p>
  
  <pre><code>Warning messages:
  1: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'BST
            '
  2: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London'
  3: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'BST
            '
  4: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London'
  </code></pre>
  
  <p>I have looked up the zoneinfo/zone.tab as per <a href="https://stackoverflow.com/questions/4047188/unknown-timezone-name-in-r-strptime-as-posixct">Joshua Ulrich's post</a> and "Europe/London" does appear in the zone.tab file, while "BST" does not. So I think that Europe/London should be a valid tz option. Is this correct?</p>
  
  <p>Does anyone have suggestions as to why I am getting warnings, and why the specified timezone is not being assigned to the as.POSIXct object?</p>
  
  <p>It should be noted that my scripts which call as.POSIXct() were running without warnings prior to updating MacOS to High Sierra. Could the OS update lead to these warnings? When I run Sys.timezone() it returns NA</p>
  
  <p>Many thanks in advance</p>
  
  <p>Iris</p>