title: Create Regular time series from an irregular (as.Date) time series with frequency=23
body: <p>I have the following problem in R. I would like to create a ts() object (i.e. a regular time series) from a irregular time series (i.e. a list of dates and data values).</p>
  
  <p>You can reproduce the problem with the following data set and R script:</p>
  
  <pre><code># dput(dd) result    
  dd &lt;- structure(list(NDVI = structure(c(14L, 4L, 11L, 12L, 20L, 17L, 
      5L, 7L, 21L, 23L, 25L, 19L, 15L, 9L, 3L, 24L, 2L, 6L, 22L, 16L, 
      13L, 18L, 10L, 8L, 1L), .Names = c("1", "2", "3", "4", "5", "6", 
      "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", 
      "18", "19", "20", "21", "22", "23", "24", "25"), .Label = c("0.4186", 
      "0.5452", "0.5915", "0.5956", "0.6010", "0.6860", "0.6966", "0.7159", 
      "0.7161", "0.7264", "0.7281", "0.7523", "0.7542", "0.7701", "0.7751", 
      "0.7810", "0.7933", "0.8075", "0.8113", "0.8148", "0.8207", "0.8302", 
      "0.8305", "0.8369", "0.9877"), class = "factor"), DATUM = structure(c(11005, 
      11021, 11037, 11085, 11101, 11117, 11133, 11149, 11165, 11181, 
      11197, 11213, 11229, 11245, 11261, 11277, 11293, 11309, 11323, 
      11339, 11355, 11371, 11387, 11403, 11419), class = "Date")), .Names = c("NDVI", 
      "DATUM"), row.names = c("1", "2", "3", "4", "5", "6", "7", "8", 
      "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", 
      "20", "21", "22", "23", "24", "25"), class = "data.frame")
  
  require(zoo)
  dd$DATUM &lt;- as.Date(dd$DATUM,"A%Y%j") # Ayear,julianday
  z &lt;- zoo(dd$NDVI,dd$DATUM,frequency=23)
  z  # this is a regular time series with a frequency=23 and start=c(2000,1)
  # there are 5 measurements in 2000 (2 jan, 1 feb, 2 apr) for which no data is available 
  # this should be marked as an NA is the final regular time series
  ts.z &lt;- as.ts(z,start=c(2000,1),frequency=23)
  </code></pre>
  
  <p>But this does not work, as I obtain a very long regular time series containing daily time steps.
  I would like to obtain a ts object with a frequency=23 correctly indicating the position for which data is not available as NA.</p>
  
  <p>I have been trying everything based on the example listed here for yearly data
  <a href="https://stackoverflow.com/questions/3897814/convert-a-irregular-time-series-to-a-regular-time-series">Convert a irregular time series to a regular time series</a></p>
  
  <p>but it does not work for data with a frequency of 23 (i.e. 23 values a year). I think I could solve it by avoiding to set <code>dd$DATUM</code> <code>as.Date()</code> but as an zoo object that can be ordered as a time series with 23 values a year.</p>
  
  <p>Any ideas?</p>
  
  <p>Thanks for your help</p>