title: Merge irregular time series data
body: <p>I have data_VOC_A and data_VOC_B are hourly data sets, however they are irregular so when I merge two data sets I want to fill the blank cells w NA. </p>
  
  <pre><code>  #data frames
  
  
          data_VOC_A &lt;-structure(list(Date = structure(1:2, .Label = c("09.09.2011 21:54", 
  "09.09.2011 22:59"), class = "factor"), hexane = c(0, 0), benzene = structure(1:2, .Label = c("0", 
  "4.4"), class = "factor"), toluene = c(2.2, 2.6)), .Names = c("Date", 
  "hexane", "benzene", "toluene"), row.names = 1:2, class = "data.frame")
  
      data_VOC_B&lt;-structure(list(Date = structure(1:2, .Label = c("09.09.2011 21:54", 
  "09.09.2011 22:59"), class = "factor"), ethane = c(14.4, 868.9
  ), propane = c(6.4, 32.1), isobutane = c(1.7, 2), n.butane = c(3.1, 
  3), isopentane = c(5.6, 3), n.pentane = c(1.4, 2.4)), .Names = c("Date", 
  "ethane", "propane", "isobutane", "n.butane", "isopentane", "n.pentane"
  ), row.names = 1:2, class = "data.frame")
  
  #merge data frames 
  d2&lt;- merge.data.frame(data_VOC_A,data_VOC_B, by='Date', all=TRUE)
  </code></pre>
  
  <p>There is a problem with the merged data frame (d2)'s timeline. </p>