I made Cerberus app some while ago when I started as a freelancer. It's vital for freelancing to track all projects, debts, etc. Sometimes a project could last several months, so you need to know how much time exactly you have in it.
The Cerberus does that for me. It does not track just debts, but my expenses and incomes as well. Everyone should have a good view of these. I made the first Cerberus version in PHP, jQuery, and jQuery-UI. That was years ago. Now I made a brand new version of this app. It's in react.js, sass, and GraphQl and the back-end is running on node.js, apollo, and MongoDB.
The app is running online, it's just a presentation with sample data, so feel free to test it.