Typical Workflow:
Engineer works on a project for a customer.
Engineer completes timesheet for hours spent on that project.
Timesheet is approved by customer's project manager / user
Approved timesheet is sent with invoice to customer finance.

So far the Zoho process assumes hours and costs are managed internally, and not approved by the customer. This is not normal. At the moment zoho books does not allow:
Approval of timesheets by customer (hardcopy is fine)
Reporting of project time per project / customer (so that other customers cannot see hours spent on other projects)

Think of the timesheet as the Delivery Note for the project.

Note, you have a previous suggestion that says timesheet approval is complete... this is not the case