Sunrise And Sunset
It is very useful for our Home Control System (HCS) to be know what time sunset and sunrise will be so that appropriate action can be taken at these times. These events can also be used to validate the detected dawn and dusk times, when using a twilight sensor.
This site shows how you can use a well defined algorithm to predict sunrise and sunset times, knowing your location, the date and current timezone. I've used a java implementation of this algorithm to enable this, though I did have to modify the code slightly to get it to fit my purpose and to handle day light saving changes.