title: How do I round a DateTime to the nearest Period
body: <p>I want to be able to round any given Instant to the nearest Period to 
  support grouping of data.</p>
  
  <p>i.e. someone might want to group by 2 week periods, so given an 
  Instant, I want to be able to work out the nearest instant (before or 
  after) on the period boundary where "now" is considered to be the end 
  of the current period. If today is Tuesday, then consider that this 2 
  week Period ends this week. Given any date (from now backward) I'd 
  like to calculate the "2 week Period" that it fits in to.</p>
  
  <pre><code>// pseudo code
  
  Period sevenDays = Days.SEVEN;
  
  Instant nearestWeek = sevenDays.roundToNearest(new DateTime());
  
  Period twelveHours = Hours.hours(12);
  
  Instant nearestHalfDay = twelveHours.roundToNearest(new DateTime());
  </code></pre>
  
  <p>I hope that makes sense, any help or pointers greatly appreciated.</p>