title: Decompose xts hourly time series
body: <p>I want to decompose hourly time series with <code>decompose</code>, <code>ets</code>, or <code>stl</code> or whatever function. Here is an example code and its output:</p>
  
  <pre><code>require(xts)
  require(forecast)
  time_index1 &lt;- seq(from = as.POSIXct("2012-05-15 07:00"),
                     to = as.POSIXct("2012-05-17 18:00"), by="hour")
  head(time_index1 &lt;- format(time_index1, format="%Y-%m-%d %H:%M:%S",
                             tz="UTC", usetz=TRUE)
  # [1] "2012-05-15 05:00:00 UTC" "2012-05-15 06:00:00 UTC"
  # [3] "2012-05-15 07:00:00 UTC" "2012-05-15 08:00:00 UTC"
  # [5] "2012-05-15 09:00:00 UTC" "2012-05-15 10:00:00 UTC"
  head(time_index &lt;- as.POSIXct(time_index1))
  # [1] "2012-05-15 05:00:00 CEST" "2012-05-15 06:00:00 CEST"
  # [3] "2012-05-15 07:00:00 CEST" "2012-05-15 08:00:00 CEST"
  # [5] "2012-05-15 09:00:00 CEST" "2012-05-15 10:00:00 CEST"
  </code></pre>
  
  <p>Why does the timezone for <code>time_index</code> change back to CEST?</p>
  
  <pre><code>set.seed(1)
  value &lt;- rnorm(n = length(time_index1))
  eventdata1 &lt;- xts(value, order.by = time_index)
  tzone(eventdata1)
  # [1] ""
  head(index(eventdata1))
  # [1] "2012-05-15 05:00:00 CEST" "2012-05-15 06:00:00 CEST"
  # [3] "2012-05-15 07:00:00 CEST" "2012-05-15 08:00:00 CEST"
  # [5] "2012-05-15 09:00:00 CEST" "2012-05-15 10:00:00 CEST"
  ets(eventdata1)
  # ETS(A,N,N) 
  # 
  # Call:
  #  ets(y = eventdata1) 
  # 
  #   Smoothing parameters:
  #     alpha = 1e-04 
  # 
  #   Initial states:
  #     l = 0.1077 
  # 
  #   sigma:  0.8481
  # 
  #      AIC     AICc      BIC 
  # 229.8835 230.0940 234.0722
  decompose(eventdata1)
  # Error in decompose(eventdata1) : 
  #   time series has no or less than 2 periods
  stl(eventdata1)
  # Error in stl(eventdata1) : 
  #   series is not periodic or has less than two periods
  </code></pre>
  
  <p>When I call <code>tzone</code> or <code>indexTZ</code> there is no timezone but the <code>index</code> clearly show that the times are defined with a timezone.</p>
  
  <p>Also, why does only <code>ets</code> work? Can it be used to decompose a time series?</p>