How do you calculate official sunset and sunrise,
We have official tables for each airport in Norway, I belive it's a national MET office product.
Google turned up this: http://aa.usno.navy.mil/data/docs/RS_OneYear.html
Input LAT/LONG and time zone down in "Form B"
I checked it with our table at work, and it's accurate to within 1-3 minutes. (But it has some occational errors as well)
The national regulations allow flight from twilight to twilight for day VFR pilots on the JAR FCL PPL.
Since it's never twilight in Trondheim in the month of june, it's possible to fly day VFR H24. It's still 3 1/2 hrs from sunset to sunrise at midsummer though. In Bergen you will need a night rating, and it's hrs between sun up/down. (If that's an option in the UK)