kimai

Kimai is the #1 open-source time-tracking application. From freelancers to companies and organisations - everyone can manage timesheets, generate reports, create invoices and so much more... Web-based multi-user application, available as On-Premise or SaaS version: https://www.kimai.org. Maintained by kimai on GitHub, where it has earned 4,813 stars from the community.

It's actively developed around doctrine, invoice, invoicing, and is a solid reference for anyone building with these tools.