A number of periodes are already predefined.
Overview:

Step 1:
Go to the screen Time/e-HR and click on the tab "Definition results". Next you click on "Periods".

Step 2:
Next you click on the icon
.
Step 3:
Add a description to your period, e.g. "five weeks".

Eventually you specify the start date that is applicable to the period.
Finally you click on
, after which this new period is created.