Wake Up Time from Rocky Sand is a desktop alarm clock that'll work like a charm as long as your Mac is awake. The app is a simple alarm clock with a handful of pleasing alarm sounds that range from "wake me up now" to "what a pleasant sound to snooze along with."
The app has simple controls with a side drawer that you can drag or double-click to open. Set the alarm time, the sound settings and get back to work. The app has a few customizations options that allow you to set the snooze length, switch to 24-hour time, and add custom alarm sounds. You also can change clock faces, if you are using the paid version.
Performance of Wake Up Time is stellar, but it has one drawback that requires mentioning. Without fail, the alarm went off as long as my Mac was awake, but when my Mac went to sleep, so did Wake Up Time alarm. Rocky Sand is aware of this limitation and has provided a helper app that'll allow Wake Up Time to wake up your Mac from sleep under certain circumstances.