I understand that sometimes people simply cannot keep track of calendars and reminders. While it can be frustrating, everyone has a different way of working and a different way of prioritizing. While you cannot force anyone to log in and sign off on their timesheets, I would enlist the help of supervisors to remind their employees of the importance of turning in the timesheets on time. At the end of the day, an employee cannot get paid if he or she does not submit paperwork to payroll, so hopefully that would be motivation enough.