It seems to be anything that would change the actions of the ones who hear it can't be passed back. I'm thinking it's a simulation that's processing 6 hours at once, with the earliest arbitrarily small unit of time being finalized at the same rate new time starts processing. So Harry just needs to upgrade the universe's hardware and he'll be good to go further back, but he should be able to get around the maximum daily uses per Time-Turner before then.

In other words:

All Cube Truth denied. 4-corner days, 24 hours divided by 4 corners is 6 hours per corner. The math is simple but no wizards will debate me. Time-Turner can only turn one corner at a time. 4 days are in one rotation. If Time-Turner turned more than 6 hours it would be in a previous day! Turners are connected in ONEness with Time and to disconnect equates death of opposites.