Options for FOSS based, self-hosted, Time Trackers.

I'm looking for an Open Source, self-hosted, timer/task tracking solution that I can use for billable time and other various categories, with decent reporting capabilities.

Using this for development projects to manage and calculate my time spent on various aspects of time spent, including billable time and other definable categories like phone support in customer interactions, writing emails, time spent on dev, testing, maintenance, security patching and updates, etc.

It would be nice if I found something that has cross platform support too, like Android and Windows clients and/or a web interface, but certainly a good self-hosted, sever based, UNIX (Linux) solution.

Ideally, the tracking system would be able to be sync'd with local offline databases (git repos or NextCloud would be awesome)when unable/inconvenient to connect with the server, and most especially, one that I can host on a server in the cloud.

Any comments, suggestions, are welcome :)