title: pandas rolling window & datetime indexes: What does `offset` mean?
body: <p>The rolling window function <a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.rolling.html" rel="noreferrer"><code>pandas.DataFrame.rolling</code></a> of pandas 0.22 takes a <code>window</code> argument that is described as follows:</p>
  
  <blockquote>
    <p><strong>window</strong> : int, or offset</p>
    
    <p>Size of the moving window. This is the number of observations used for
    calculating the statistic. Each window will be a fixed size.</p>
    
    <p>If its an offset then this will be the time period of each window.
    Each window will be a variable sized based on the observations
    included in the time-period. This is only valid for datetimelike
    indexes. This is new in 0.19.0</p>
  </blockquote>
  
  <p>What actually is an <em>offset</em> in this context?</p>