I have this simple program:
DateTime aux = new DateTime(2012, 6, 12, 12, 24, 0); DateTime aux2 = new DateTime(2012, 6, 12, 13, 24, 0);
DateTime.AddDays returns new DateTime that adds specified number of days. You need to assign it to your variable:
DateTime.AddDays
aux2 = aux2.AddDays(1);