title: R: as.POSIXct timezone and scale_x_datetime issues in my dataset
body: <p>I spent some time trying to figure out why the hour ticks were shifted when scale_x_datetime was applied. I've tried to give the timezone when the Date/Time column was created. I used ggplot and scale_x_datetime() from the package scales. The hour ticks were wrong, which datapoint did not match the time in their Date/Time column.</p>
  
  <p>Here is some procedures to deal with my dataset. </p>
  
  <pre><code>  DF$DateTime&lt;-as.POSIXct(DF$timestamp,format="%m/%d/%y %H:%M", tz="America/Toronto")
    DF$Date&lt;-as.Date(DF$DateTime)
  
    lims &lt;- as.POSIXct(strptime(c("2015-07-21 00:00","2015-07-23 00:00"), format = "%Y-%m-%d %H:%M"), tz="America/Toronto")    
  
    ggplot(DF) + geom_line(aes(x=DateTime, y=-Diff,group=Date)) + scale_x_datetime(limits =lims, breaks=date_breaks("2 hour"), labels=date_format("%m/%d %H:%M"))
  </code></pre>
  
  <p>Do I miss anything here?? Please help me to figure it out.
  Many thanks!</p>