Hi Rigpapa,
I think I got it now. I was stuck on the Vera trigger idea too much and your conditions are actually states (better as it will still work when a reload happens during the time change). Makes no difference for a value (Light is on or off), but does for a particular time of day. So I now set it for after Dusk with an offset of -15 minutes and that seems to be doing what I expect.
Great work.
Cheers Rene