title: Time series, change monthly data to quarterly
body: <p>Now I have some monthly data like :</p>
  
  <pre><code>1/1/90 620
  2/1/90,591
  3/1/90,574
  4/1/90,542
  5/1/90,534
  6/1/90,545
  #...etc
  </code></pre>
  
  <p>If I use ts() function, it's easy to make the data into time series structure like:</p>
  
  <pre><code>         Jan Feb Mar ... Nov Dec
     1990  620 591 574 ... 493 464
     1991  100 200 300 ...........
  </code></pre>
  
  <p>Is there any possibilities to change it into quarterly repeating like this:</p>
  
  <pre><code>            1st 2nd 3rd 4th
     1990-Q1  620 591 574 464
     1990-Q2  100 200 300 400
     1990-Q3  ...
     1990-Q4  ...
     1991-Q1  ...
  </code></pre>
  
  <p>I tried to change </p>
  
  <pre><code>   ts(mydata,start=c(1990,1),frequency=12)
  </code></pre>
  
  <p>to</p>
  
  <pre><code>   ts(mydata,start=c(as.yearqrt("1990-1",1)),frequency=4) 
  </code></pre>
  
  <p>but it seems not working.</p>
  
  <p>Could anyone help me? Thank you very much. </p>