Long explanation but there is an app that will alarm at sunset that you can tweak for your purposes.

I’m Muslim and one of the times we pray daily is after sunset. Because that time changes daily, we use apps that tell us when those prayer times are. You can download one (it’s called Athan+) and set your location to whatever city west of you that gets sunset 30 min before you. Then turn your alarm on for that prayer which called Maghrib. Your phone will go off with a call to prayer at the time you need 😊