There are two settings that controls how much backwards your users can enter timesheets:

1. Profile Settings: Login as Administrator and got to Timesheet > Admin > Profiles, then under Functions, set the backward and forward periods.
   
   
2. Payroll Date: Timesheet cannot be entered before the payroll start date. To change it go to Timesheet > Admin > Users, select the user then scroll down to the Payroll section and change the payroll start date.