If you need to see statistics for previous or next date range, you can achieve that by single click to arrow in the period bar:
New date range has the same number of days with exception when first and last day of range is first and last day of the month. In that case new range is also aligned to month boundary so you can change months and years easily.
If you wish to see statistics of two periods concurrently, you can use feature described here: http://www.niftystats.com/blog/2011/06/08/version-period-comparison-released/
