Corrected entry: How would the robbers know exactly what time the lights were going on at the McCallisters' house? It is the first evening they are away from home; and even though Harry visited the house the evening before, he was not told the exact second when the timers were set and didn't leave the hallway to find out for himself.
LorgSkyegon
11th Mar 2013
Correction: They've been casing the neighborhood and Harry made notes on exactly when the automatic lights came on. The people in the neighborhood would have the timers set all the time, not just when they're away.
LorgSkyegon