Tie Tracker is a free and open source time tracking application. Its is meant to be simple and aims to allow you to track working hours across projects.
» Features
Track your productivity and billable hours.
* Simple work hours tracking
* Assign time to clients and projects
* Mark entries as billed
» Reporting
Yet not complicated but effective reporting.
* Weekly work summary
* Daily list of activities
* Export open invoices to XLSX timesheets (Excel, LibreOffice, Numbers, etc) and PDF