You probably know how you can compare data between weeks, month etc. and how Disconnected Date Dimension work (if not, read this). But maybe you don't know how to compare user specified period of time against the same range of dates before this selected period. And that's where the DDD knowledge mentioned before comes in.
Imagine that you want to compare four selected days against previous four days (to follow one of the example screenshots below, it could be 2014/06/25 - 2014/06/28 against 2014/06/21 - 2014/06/24). It's not any usual period of time (like week or month), so you can't use something like FOR PreviousPeriod.
To solve this issue just follow these steps:
And that's it. You can choose i.e. 247 days to compare, it doesn't metter. Eat that, time comparisons!