Many handheld GPS's will tell you the local sunrise sunset times. I'm certain that you could not be found wrong for flying with reference to this information.
When they say "horizon", is that the one in front of you while sailing across the ocean, or the one beside you while you take of in a mountain valley. I've had to look up at a 40 degree angle to see the horizon while flying in the mountains.
And the proof.. Aside from your radio call to ATC, or transponder return recorded along with a time, I think that the authorities would have hard time proving your takeoff time with precision equal to that of the positon of the sun as described.
Funny that they'll define sunset with such precision, and yet not define "icing" at all (well in Canada anyway)!
Pilot DAR