title: Unexpected behavior resampling time series
body: <p>I am attempting to resample tick time series data in Pandas.</p>
  
  <pre><code>time
  2014-07-01 10:00:00    1476.29
  2014-07-01 10:00:01    1474.70
  2014-07-01 10:00:02    1475.28
  2014-07-01 10:00:02    1474.65
  2014-07-01 10:00:04    1474.64
  2014-07-01 10:00:05    1474.15
  ...
  </code></pre>
  
  <p>I use resample</p>
  
  <pre><code>data["2014-07-01 10:00:20":].resample("15s")
  </code></pre>
  
  <p>and get something like this:</p>
  
  <pre><code>time
  2014-07-01 10:00:15    1472.711000
  2014-07-01 10:00:30    1472.376000
  2014-07-01 10:00:45    1471.965333
  2014-07-01 10:01:00    1470.182667
  2014-07-01 10:01:15    1469.607333
  2014-07-01 10:01:30    1469.746429
  ...
  </code></pre>
  
  <p>I don't understand why it starts with "2014-07-01 10:00:15" while I resample slice starting with "2014-07-01 10:00:20". Is it a feature or a bug? How can I make it start with needed time? </p>
  
  <p>So what I need is to compute new time series with values of standard deviation of n previous seconds in given data. For doing this I decided to split it in pieces starting with certain time and then combine them. Maybe there is a better way to do this? Don't want to use iterrows().</p>