title: Subset ZOO Object by datetime
body: <p>I'm trying to subset the the ZOO object x below for SaleDateTime > "2013-01-01 00:00:00.00".  I've tried the attempt below, which get close but not exactly what I want.  For some reason it returns records starting in 2014.  The data goes all the way back to 2011 and there's plenty of values with SaleDateTime in 2013.  The data has the form SaleDateTime = '2012-02-01 23:00:00.000' and SaleCount=4.  This doesn't seem like it should be that tricky.  I would definitely appreciate advice.</p>
  
  <pre><code>x&lt;-zoo(Value$SalesCount, order.by=Value$SaleDateTime)
  </code></pre>
  
  <p>Attempt 1</p>
  
  <pre><code> vv&lt;-subset(x, index(x)&gt;=as.Date("2013-01-01 00:00:00.000"))
  </code></pre>
  
  <p>Warning message:
  In eval(expr, envir, enclos) :
    Incompatible methods ("Ops.factor", "Ops.Date") for ">="</p>