Location information is essential when discovering many date and time related laws, customs and calculations.
Unlike the civil calendar in which one day becomes the next at the stroke of midnight, the transition from one day to the next in the Jewish calendar occurs somewhere between sunset and nightfall. Therefore the Jewish date will always overlap with two days in the civil calendar, and the same is true of the reverse.
This is the reason we ask for the city where an event took place to determine when nightfall occurred on the date you selected (which depends, of course, on where it occurred). Once we do this we can determine whether an event took place in the first or second of the two Jewish dates corresponding to the civil date.
If an event took place sometime between sunset and nightfall please consult a Rabbi to determine what custom you should follow in this respect.
