You only need to look out of the window - if its dark its night and vice versa. Unless you need a certain number of night hours for a rating no-one gives a stuff what you put into your log-book so don't worry about it ! If I fly a sector that involves night flying I just make a rough approximation when I scribble the flight time in my diary - simple as that and usually worked out ot the nearest fifteen minutes - makes totalling up so much easier.