Almost all the big organizations use timesheet to track the work of employees. In this sheet, employees fill the number of hours they work and submit it for approvals. This happens at the end of every week. This way company can easily track about the number of hours of work done by employees.
If you are small organization or freelancer then you can make use of online timesheet generator service known as WeWorked. This is easy to use site where you can fill your data and update it every week. All you need to do is sign up for your account and start using this service.
There is option to create your company profile add users. Other users can login in the service and fill their timesheet and send for final approvals. It also sends automatic reminders to users so that they do not miss filling time sheet.
This is a perfect tool for auditing purpose also. It gives you detailed reports of users. You can easily track number of hours of work performed by employees. This tool is free to use.
Check out WeWorked.