The date picker can be found at the very top of the page:

It consists from two fields:

  • Period Type
  • Period

The Period Type is the length of time that ranges from a Day to a Year:

Once you pick the Period Type, you can then define the Period itself, usually in the form of This "Period" and Last "Period" - as in This Month/Last Month, or "This Year/Last Year": 

The date picker changes to a slightly modified version when the dashboard is in Compare Mode and allows for comparison two different time periods: