title: Can't index xts class with Date type
body: <p>I'm loading some data manually (vs. via quantmod) and trying to create an xts class (which all seems to work ok), but I can't seem to use Date type indexes.</p>
  
  <p>I am trying to find the intersection with another time series (NDX, via quantmod), which should be easy if I could use date indexes. </p>
  
  <p>I'm assuming I'm missing something as a create the xts object but at a loss to know what.## Heading ##</p>
  
  <pre><code>VXN.tmp &lt;- read.csv('VXNDailyPrices.csv', na.strings=c('n/a'), stringsAsFactors=F, header=F)
  VXN &lt;- xts(as.matrix(VXN.tmp[,-1]), as.Date(VXN.tmp[,1], tz="", format='%m/%d/%y'), tzone="")
  colnames(VXN) &lt;- c('Open', 'High', 'Low', 'Close')
  VXN &lt;- VXN[!is.na(Cl(VXN))]
  
  &gt; head(VXN)
            Open High Low Close
  2001-02-02   NA   NA  NA 54.89
  2001-02-05   NA   NA  NA 55.85
  2001-02-06   NA   NA  NA 53.68
  2001-02-07   NA   NA  NA 54.41
  2001-02-08   NA   NA  NA 54.66
  2001-02-09   NA   NA  NA 55.85
  &gt; VXN['2001-02-02']
             Open High Low Close
  2001-02-02   NA   NA  NA 54.89
  &gt; VXN[as.Date('2001-02-02')]
       Open High Low Close
  &gt; 
  </code></pre>
  
  <p>Some other info</p>
  
  <pre><code>&gt; str(VXN)
  An ‘xts’ object from 2001-02-02 to 2013-03-01 containing:
    Data: num [1:3040, 1:4] NA NA NA NA NA NA NA NA NA NA ...
   - attr(*, "dimnames")=List of 2
    ..$ : NULL
    ..$ : chr [1:4] "Open" "High" "Low" "Close"
    Indexed by objects of class: [Date] TZ: 
    xts Attributes:  
  List of 2
   $ tclass: chr "Date"
   $ tzone : chr ""
  &gt; 
  &gt; vxnidx &lt;- head(index(VXN), 20)
  &gt; dput(vxnidx)
  structure(c(11355, 11358, 11359, 11360, 11361, 11362, 11365, 
  11366, 11367, 11368, 11369, 11373, 11374, 11375, 11376, 11379, 
  11380, 11381, 11382, 11383), class = "Date")
  &gt; 
  &gt; vxnidx[1]
  [1] "2001-02-02"
  &gt; VXN[vxnidx[1]]
       Open High Low Close
  &gt; VXN["2001-02-02"]
             Open High Low Close
  2001-02-02   NA   NA  NA 54.89
  &gt;
  </code></pre>
  
  <p>The data is from the CBOE here: <a href="http://www.cboe.com/micro/vxn/#historical" rel="nofollow">http://www.cboe.com/micro/vxn/#historical</a></p>
  
  <pre><code>2/2/2001,n/a,n/a,n/a,54.89
  2/5/2001,n/a,n/a,n/a,55.85
  2/6/2001,n/a,n/a,n/a,53.68
  2/7/2001,n/a,n/a,n/a,54.41
  2/8/2001,n/a,n/a,n/a,54.66
  2/9/2001,n/a,n/a,n/a,55.85
  2/12/2001,n/a,n/a,n/a,57.05
  2/13/2001,n/a,n/a,n/a,58.42
  2/14/2001,n/a,n/a,n/a,57.49
  2/15/2001,n/a,n/a,n/a,55.19
  2/16/2001,n/a,n/a,n/a,55.34
  2/20/2001,n/a,n/a,n/a,57.93
  2/21/2001,n/a,n/a,n/a,59.57
  2/22/2001,n/a,n/a,n/a,60.99
  2/23/2001,n/a,n/a,n/a,62.03
  2/26/2001,n/a,n/a,n/a,61.77
  2/27/2001,n/a,n/a,n/a,63.27
  2/28/2001,n/a,n/a,n/a,63.65
  3/1/2001,n/a,n/a,n/a,64.32
  </code></pre>
  
  <p>edit: session info</p>
  
  <pre><code>&gt; sessionInfo()
  R version 2.15.2 (2012-10-26)
  Platform: i386-apple-darwin9.8.0/i386 (32-bit)
  
  locale:
  [1] en_AU.UTF-8/en_AU.UTF-8/en_AU.UTF-8/C/en_AU.UTF-8/en_AU.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] PerformanceAnalytics_1.0.4.4 quantmod_0.3-17              TTR_0.21-1                  
  [4] xts_0.8-6                    zoo_1.7-7                    Defaults_1.1-1              
  
  loaded via a namespace (and not attached):
  [1] grid_2.15.2     lattice_0.20-10
  &gt; 
  </code></pre>