The quickest way to change the dates you are viewing in Phocas is to choose from a list of already defined periods. Defined periods contain a current period and previous period and are usually dynamic, meaning they adjust according to today's date.
Select a defined period from the list (in the image below, Financial Q1 is being selected).
Check what dates are in a period
To check what dates a particular defined period refers to, choose a period (e.g., Period > Last month) and when the grid updates select Period > Custom. The dialog that opens will show the dates that were set for this defined period.