Hey, I know that this is an "ancient" post, but maybe it would be nice to throw a solution here just for the sake of completeness and in case someone may stumble upon it in the future.
Nowadays you can use BI's function BIS_fnc_sunriseSunsetTime to get the sunrise and sunset of the given date. There is an example in the documentation
webpage but I'll duplicate it (modified) here for completeness.
private _riseSet = date call BIS_fnc_sunriseSunsetTime; // Get both sunrise and sunset
systemChat format["The sun rises at %1 and sets at %2", _riseSet select 0, _riseSet select 1]; // Print them out
Hope this helps someone in the future.