The error You can't create a Timesheet before the start date of the System occurs when the Effective date on Timesheet period type is different from Start date set on User's account.

Resolution:
  • Login with Administrator Privileges.
  • Click on Administration icon seen in the System section at the top.
  • Click Timesheet period under Timesheet setup in the left pane of the screen.
  • Click Edit to delete the Timesheet period type.  Click Yes to confirm.
  • Click Add to select Timesheet Period Type, First Day of Week and Effective Date.
  • Click Save
Note:
The Timesheet period effective date should always be earlier than the start date set on User's account, if time had to entered retrospectively.