Employees use the Mobile Time Tracker application and track hours against Projects/Jobs using their smartphones, in Salesforce, or from the Mobile Time Tracker Web app.
The total hours worked by the employee are stored in the Mobile Time Tracker application, at the end of the pay period, you can click the "Generate Invoices" button in the TT setup screen or you can create a scheduler to automatically create the Invoices in Xero by following the instructions here.
simply click to sync that time data with Xero. Voila! Now you have an accurate-to-the-second record of employee hours that can be synced with Xero and used to run payroll, job cost, create invoices, and bill clients.