Looking for a decent time-tracking application for Mac

I am a freelance graphic designer and need to track the time spent on projects so I can bill my clients accordingly. There are a *ton* of services and applications out there but somehow none of them really fits. Here's what I am looking for: - A stable desktop app. I don't want to do my time tracking in the browser. - Stop recording when screensaver or sleep mode starts. - A product I can buy, not a subscription service. I feel more comfortable and in control if I pay $100 upfront than pay $10 a month - An easy-to-use UI that tracks time by client, project and task. - Billing support is a bonus, but not necessary. Here is what I don't need, so if the solution contains it, at least is shouldn't interfere with a simple workflow and efficiency: - Not necessary: Smartphone app, team collaboration, cloud. All I want is a pay-once, stand-alone, well-supported and nicely designed time-tracking app. Do products exist in this space for OSX?