[sschueller]

Why not let the user pick the 3 hour window and not let them change it for another period after it was just changed?

[diggan]

Or, have N available 3 hour windows, and if you've interacted (viewed the website/posted) in any way with the website during one of those periods, you cannot use the other periods for that day.

So basically the same idea, but letting the decision be more dynamic.