There is a few different concepts in the astro binding that you have to separate:
You have start and end times, for example for the sun set, that would be astro:sun:local:set#start, which gives you the time that the sun starts to set, and astro:sun:local:set#end, which gives you the time that the sun set ends.
Furthermore you have durations, that just gives you the number of minutes that the sun set takes, and you have a range event, which triggers a START and END at the respective times. It is that range event that you would typically use in a rule:
rule "Do something when sun set starts"
Channel "astro:sun:local:set#event" triggered START
// do something