Hello

There are two different options, you already mentioned them I think.

1) You are going to make a product, that shall be sold elsewhere in the world.

Then you may want to configure the system with latitude and longitude, and let an algoritm calculate the data. Also take into account that beyond the polar circle there may be no sunrise or sunset for several days at all; but maybe twiligth for a few hours, depending on the location (I've been there, I've seen it).

However, it is more likely that this aplies for your problem:

2) One product for a known and fixed location.

Just make a small table of 365 entries for sunrise and sunset for this particular location. And do not worry about february 29th, use data of 28th.

There accumulates an small error of approx. 4 minutes every 4 years; until the next february 29th, but this is for your application (switching lamps) not relevant.

I use solution number 2 in my home automation system for a long time now and I'm happy with it (no floating point at all). It took me an hour to type the numbers in, that's all, no difficult algorithms to test and verify.

Best regards
Juergen