title: as.Date conversion returns NA
body: <p>I am using <a href="http://www.abs.gov.au/AUSSTATS/abs@.nsf/DetailsPage/6416.0Mar%202017?OpenDocument" rel="nofollow noreferrer">this data</a>. When I want to define the variable <code>as.Date()</code> I am getting <code>NA</code>.</p>
  
  <p>This is the code I am using. Can someone please advise what I am doing wrong? </p>
  
  <pre><code>dataf &lt;- read.csv("DB.csv")
  dataf$Date &lt;- as.Date(dataf$Date, format = "%b-%Y")
  str(dataf)
  'data.frame':   55 obs. of  9 variables:
   $ Date     : Date, format: NA NA ...
   $ Sydney   : num  85.3 88.2 87 84.4 84.2 84.8 83.2 82.6 81.4 81.8 ...
   $ Melbourne: num  60.7 62.1 60.8 60.9 60.9 62.4 62.5 63.2 63.1 64 ...  
   $ Brisbane : num  64.2 69.4 70.7 71.7 71.2 72 72.6 73.3 73.6 75 ...
   $ Adelaide : num  62.2 63.9 64.8 65.9 67.1 68.6 68.6 69.3 70 71.6 ...  
   $ Perth    : num  48.3 50.6 52.5 53.7 54.7 57.1 59.4 62.6 65.2 70 ...
   $ Hobart   : num  61.2 66.5 68.7 71.8 72.3 74.6 75.8 76.9 76.7 79.1 ...
   $ Darwin   : num  40.5 43.3 45.5 45.2 46.8 49.7 53.6 54.7 56.1 60.2 ...
   $ Canberra : num  68.3 70.9 69.9 70.1 68.6 69.7 70.3 69.9 70.1 71.7 ...
  </code></pre>