title: How to change character string into yearqtr
body: <p>I have a dataseries which looks like:</p>
  
  <pre><code>gdp &lt;- structure(c(4.61992172242418, 4.6566625355851, 4.36946546173971, 
  4.84162425714203, 5.27383573389955, 5.13243064162054, 4.95781801538584, 
  5.26294884122223, 12.5001588735103, 14.5451731219964, 10.3624546107312, 
  10.3243661354426), .Dim = c(4L, 3L), .Dimnames = list(NULL, c("in.gdp.fc", 
  "in.gdp.real", "in.gdpnagri")), index = c("Apr-Jun 2012", 
  "Jul-Sep 2012", "Oct-Dec 2012", "Jan-Mar 2013"), class = "zoo")`
  </code></pre>
  
  <p>How can I change the index into 'yearqtr' class? I tried doing as.Date and then as.character.
  It is throwing up some error saying: Error in charToDate(x) : 
    character string is not in a standard unambiguous format.</p>
  
  <p>Can someone please help?</p>