It depends on whether your temporal logic is based on a relationist or an absolutist position. If you are to accept the notion that 'time does not exist without change' - as Aristotle argued - then 'evening' may be observed as starting only in relation to the end of 'afternoon'. In other words, one could not impose a pre-determined start time for 'evening' , instead only describe it as occurring when 'afternoon' has elapsed. On the other hand, a Newtonian analysis would dictate that time exists independently of the objects or actors within. That is to say '5 for 5:30'