title: Harmonise date format with r
body: <p>I have a file with multiple data formats :</p>
  
  <pre><code>date&lt;-structure(list(date = structure(c(10L, 45L, 40L, 24L, 53L, 51L, 
  13L, 68L, 29L, 11L, 40L, 47L, 52L, 34L, 39L, 46L, 20L, 15L, 26L, 
  72L), .Label = c("", "01/2005", "01/2015", "03/2008", "05/2008", 
  "05/2012", "06/2014", "08/2008", "09/2010", "1/12/2001", "1/2002", 
  "10/2007", "10/2010", "10/2011", "11/2013", "12/2004", "12/2006", 
  "12/2010", "12/2014", "15/02/2014", "16/04/2014", "16/12/2014", 
  "17/07/2006", "1968", "1979", "1982", "1984", "1985", "1986", 
  "1992", "1993", "1994", "1997", "1998", "1998/02", "2/2008", 
  "2/8/2000", "2000", "2001", "2002", "2002/08", "2003", "2004", 
  "2005-01-01", "2006-06-01", "2007", "2007-12-20", "2008", "2008-06-28", 
  "2009", "2010", "2011", "2012", "2012/03", "2013", "2014", "21/10/2014", 
  "23/10/2014", "24/2000", "3/2007", "5/2009", "6/2001", "6/2012", 
  "8/5/1993", "9/2009", "April 2012", "Jan 1975", "Jan 1976", "Mar 1976", 
  "May 1976", "Nov 18, 1975", "Sep 10, 1976"), class = "factor")), .Names = "date", row.names = c(NA, 
  -20L), class = "data.frame")
  </code></pre>
  
  <p>What I would like to do is extract the year part of these dates.</p>
  
  <p>How can I do this? My idea was to put all dates in a standard format then extract the year with a subsrtring. However, I tried to standardise the dates using <code>as.POSIXct</code>but given the lack of a standard format this doesn't work.</p>