title: How to create "NA" for missing data in a time series
body: <p>I have several files of data that look like this:  </p>
  
  <pre><code>X code year month day pp  
  1 4515 1953     6   1  0  
  2 4515 1953     6   2  0  
  3 4515 1953     6   3  0  
  4 4515 1953     6   4  0  
  5 4515 1953     6   5  3.5
  </code></pre>
  
  <p>Sometimes there is data missing, but I don't have NAs, the rows simply don't exist. I need to create NAs when the data is missing. I though I could start by identifying when that occurs by converting it to a zoo object and check for strict regularity (I never used zoo before), I used the following code:   </p>
  
  <pre><code>z.date&lt;-paste(CET$year, CET$month, CET$day, sep="/")
  z &lt;- read.zoo(CET,  order.by= z.date )
  reg&lt;-is.regular(z, strict = TRUE)
  </code></pre>
  
  <p>But the answer is always true! </p>
  
  <p>Can anyone tell me why is not working? Or even better, tell me a way to create NAs when the data is missing (with or without zoo package)?</p>
  
  <p>thanks</p>