Assume they are in a simulation - why would it have an event buffer created able to compute time travel at all, and why pick 6 human hours (equivalent) as the magic number constant for it?

It occurs that magic is basically the ability to hack the simulation; the wizards who developed time travel didn't know what was actually safe, what was unsafe, what would crash the simulation, and what the simulation could actually even do - so they picked the weakest version of time travel to implement (both branches consistent) and slapped a bunch of arbitrary limits on it so it (hopefully) couldn't break anything major.