title: Merge and join with Daylight Saving
body: <p>I am having problems merging / joining data for the coming daylight savings shift. My time-vector <code>d</code> is supposed to be the controlling time-vector, so when I join with data with missing holes I just get NA values. This normally works brillantly. However, during the coming <code>'2015-10-25 02:00:00'</code> it goes horribly wrong.  </p>
  
  <p>Data example: </p>
  
  <pre><code>d &lt;- seq.POSIXt(from = as.POSIXct("2015-10-25 00:00:00", "%Y-%m-%d %H:%M:%S", tz = ""), 
                  to = as.POSIXct("2015-10-25 23:00:00", "%Y-%m-%d %H:%M:%S", tz = ""), by = "hour")
  
  df1 &lt;- data.frame(Date = d, value1 = 1:25)
  
  df2 &lt;- data.frame(Date = as.POSIXct(format(d, "%Y-%m-%d %H:%M:%S"), tz = ""), value2 = 26:50)
  
  require(dplyr)
  
  df &lt;- left_join(df1, df2, by = "Date")
  
  df &lt;- merge(df1, df2, by = "Date", all.x = TRUE)
  </code></pre>
  
  <p>Both <code>left_join</code> and <code>merge</code> gives wrong results, and I am not sure what goes wrong. Well, I can see R has no idea how to handle the two repeated hours - and that is completely understandable. Both time series are <code>POSIXct</code>, but there is clearly some information I am missing? How can you handle this? I would prefer a base R-solution.  </p>
  
  <p>It gets exponentially worse, if you need to do even more joins from different data-sets. I need to join 7 and it just gets worse and worse. </p>
  
  <p>The correct result is:</p>
  
  <pre><code>result &lt;- data.frame(Date = d, var1 = df1[, 2], var2 = df2[, 2])
  </code></pre>