I'd abolish many time related scenarios, leaving it to references like morning, midday, and evening at most. In a roleplay it doesn't really matter if in the time someone bakes a pie that another character flew across the Atlantic unless you reference it. What matters is the post and the event that took place in that message. Some RPs have an endless night and don't break into daylight, and it works cause of the way it's written. 

Most tabletop rpgs do this right. They create a scenario and the players play out those events without referencing time. It doesn't distract from the RP and the only situations that bring up time are when sight and hearing need to be used.

The problems with sticking to a time slot is that you are micromanaging the players post. They need to calculate how much a task takes and often it detracts from what is written. Sure it works sometimes, but it's too much effort and many people won't stick to it well.