sunrize and sunset are given by the satelites / GPS, iGo switches analog to this information. A setting for this would not make sence, since these times vary through out the year and from location to location. People in the northern part of the northern hemisphere and analog south, have longer dusk- and dauwn-times, the actual sunset / sunrize times are earlyer / later... To make a long story short, if you are really keen on that, you will have to change the clock.
I can just confirm this in parts - I have the same problem with summertime, the unit always switches 1 or 2 hours too early!! And if this is controlled by GPS-signal, there must be a setting somewhere that tells iGo, when to switch!
And if there is a setting, this setting can be changed, I guess!!