title: cbind() is changing date formatting
body: <p>I'm trying to create a subset of a data frame and when I do so, R switches the formatting of the date column.  Any idea why or how to fix this?</p>
  
  <pre><code>&gt; head(spyPr2)
          Date   Open   High    Low  Close    Volume Adj.Close
  1 12/30/2011 126.02 126.33 125.50 125.50  95599000    125.50
  2 12/29/2011 125.24 126.25 124.86 126.12 123507200    126.12
  3 12/28/2011 126.51 126.53 124.73 124.83 119107100    124.83
  4 12/27/2011 126.17 126.82 126.06 126.49  86075700    126.49
  5 12/23/2011 125.67 126.43 125.41 126.39  92187200    126.39
  6 12/22/2011 124.63 125.40 124.23 125.27 119465400    125.27
  &gt; spyPr2$Date &lt;- as.Date(spyPr2$Date, format = "%m/%d/%Y")
  &gt; head(spyPr2)
          Date   Open   High    Low  Close    Volume Adj.Close
  1 2011-12-30 126.02 126.33 125.50 125.50  95599000    125.50
  2 2011-12-29 125.24 126.25 124.86 126.12 123507200    126.12
  3 2011-12-28 126.51 126.53 124.73 124.83 119107100    124.83
  4 2011-12-27 126.17 126.82 126.06 126.49  86075700    126.49
  5 2011-12-23 125.67 126.43 125.41 126.39  92187200    126.39
  6 2011-12-22 124.63 125.40 124.23 125.27 119465400    125.27
  &gt; spyPr2 &lt;- data.frame(cbind(spyPr2$Date, spyPr2$Close, spyPr2$Adj.Close))
  &gt; str(spyPr2)
  'data.frame':   1638 obs. of  3 variables:
   $ X1: num  15338 15337 15336 15335 15331 ...
   $ X2: num  126 126 125 126 126 ...
   $ X3: num  126 126 125 126 126 ...
  &gt; head(spyPr2)
       X1     X2     X3
  1 15338 125.50 125.50
  2 15337 126.12 126.12
  3 15336 124.83 124.83
  4 15335 126.49 126.49
  5 15331 126.39 126.39
  6 15330 125.27 125.27
  </code></pre>
  
  <p>UPDATE:</p>
  
  <pre><code>&gt; spyPr2 &lt;- data.frame(cbind(spyPr2["Date"], spyPr2$Close, spyPr2$Adj.Close))
  Error in `[.data.frame`(spyPr2, "Date") : undefined columns selected
  &gt; spyPr2 &lt;- data.frame(cbind(spyPr2[,"Date"], spyPr2$Close, spyPr2$Adj.Close))
  Error in `[.data.frame`(spyPr2, , "Date") : undefined columns selected
  </code></pre>
  
  <p>UPDATE 2:</p>
  
  <pre><code>structure(list(Date = structure(c(15338, 15337, 15336, 15335, 
  15331, 15330), class = "Date"), Open = c(126.02, 125.24, 126.51, 
  126.17, 125.67, 124.63), High = c(126.33, 126.25, 126.53, 126.82, 
  126.43, 125.4), Low = c(125.5, 124.86, 124.73, 126.06, 125.41, 
  124.23), Close = c(125.5, 126.12, 124.83, 126.49, 126.39, 125.27
  ), Volume = c(95599000L, 123507200L, 119107100L, 86075700L, 92187200L, 
  119465400L), Adj.Close = c(125.5, 126.12, 124.83, 126.49, 126.39, 
  125.27)), .Names = c("Date", "Open", "High", "Low", "Close", 
  "Volume", "Adj.Close"), row.names = c(NA, -6L), class = "data.frame")
  </code></pre>