You need to specify the vat start period and the vat end period.
You can do this by simply clicking onto the start period date via the calendar.
Either double click on the date you want to use or click it once, and then click the Use button.
To abort click the Cancel button.
TIP
By default the system will automatically set the end period to 3 months after the start period, although you can change this yourself.
Also be aware, if 'You are in TEST MODE' then the dates will be changed to the predefined dates, regardless of the date you pick.