1)If you are unable to change the backdated timesheet before the restricted date then follow the following step:
Goto –> Tcode CAC1 and change the lower limit to the required backdated weeks. This should work unless something is hard coded.
2)If still it is not working then GOTO –>Tcode SMOD –> CATS0003 –> Click on component –> chk the user exit –> check if time sheet data is hard coded to limit the number of days to 90 days backdated to save the time sheet data.
This would definelty help