title: Extracting hourly data from 15 minute data
body: <p>I have a data frame with two columns, time and flow. The time interval for the time series is 15 minutes and I want to cut this time series so that the output time series has consistent one hour time intervals and the flow value from that hourly time stamp from the original data. How do I extract the hourly data?</p>
  
  <p>Input:</p>
  
  <pre><code>structure(list(t = structure(c(1104555600, 1104556500, 1104557400, 
  1104558300, 1104559200, 1104560100, 1104561000, 1104561900, 1104562800
  ), class = c("POSIXct", "POSIXt"), tzone = "EST"), flow = c(18, 
  18, 18, 18.125, 18.125, 18.125, 18.125, 18.125, 18.125)), .Names = c("t", "flow"), row.names = c(NA, 9L), class = "data.frame")
  </code></pre>
  
  <p>And for output I would want something like</p>
  
  <pre><code>time    flow
  2005-01-01 00:00:00 18.000
  2005-01-01 01:00:00 18.125
  2005-01-01 02:00:00 18.125
  </code></pre>