How Long Left counts down your current calendar events. If you're in a meeting, How Long Left can tell you "Meeting ends in 5 minutes." It is currently avaliable as a macOS app, and an iOS/watchOS app.
As long as you have the latest version of Xcode installed, you can simply clone this repo using the Terminal command:
git clone https://github.com/ryankontos/How-Long-Left
This will place a copy of the How Long Left source code in your Home directory. You can then launch How Long Left.xcworkspace, set unique Bundle Identifiers and build.