title: convert string data.frame to Date
body: <p>I am wondering why this error occurs.  I would like to convert this using brackets as I am making sequential conversions in a loop. And because I just want to be able to do it and understand what is happening. </p>
  
  <pre><code>head(clean.deposit.rates)
         Date
  1 1/31/1983
  2 2/28/1983
  3 3/31/1983
  4 4/30/1983
  5 5/31/1983
  6 6/30/1983
  
  class(clean.deposit.rates)
  [1] "data.frame"
  
  class(as.Date(clean.deposit.rates[[1]], "%m/%d/%Y"))
  [1] "Date"
  
  
  
      class(as.Date(clean.deposit.rates$Date, "%m/%d/%Y"))
  [1] "Date"
  
  
  
   as.Date(clean.deposit.rates["Date"], "%m/%d/%Y")
      Error in as.Date.default(clean.deposit.rates["Date"], "%m/%d/%Y") : 
        do not know how to convert 'clean.deposit.rates["Date"]' to class “Date”
  </code></pre>