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:
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Login with Administrator Privileges.
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Click on Administration icon seen in the System section at the top.
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Click Timesheet period under Timesheet setup in the left pane of the screen.
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Click Edit to delete the Timesheet period type. Click Yes to confirm.
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Click Add to select Timesheet Period Type, First Day of Week and Effective Date.
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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.