The time of the day is set by default on 24h days, starting at around 18h14, which means that you have 1h46 to complete daytime. It is this way because sunset has way less traffic than midday, but the game doesn't count the preset times 'Sunrise' and 'Sunset' as daytime, so you have to manually set the game to 6pm in order to get a time that is both accepted as daytime and considered 'sunset'. Since you cannot freeze a custom time of the day, 24h days is the only option.