@sunset/@sunrise are driven by the weather underground APP. If you have no internet, then they will only be as accurate as the last day you had internet. Depending on the time of the year, this could be a few seconds or a few minutes difference. If you have no internet at all, those won't work.
The alternative is to use the excellent jkUtils Solar Altitude APP. It has quite a few options for both sunrise and sunset that give you a finer level of control over just how dark you want things to be when they run. In the Advanced Smart Lights code I originally used WU but opted for jkUtils when jens released it so that I wouldn't be reliant on the internet in order for my lights to work.