I believe that rules like this should be rephrased so that you are asked to wait as long as possible without missing your turn in a scenario in which you are supossed to move after an opponant, and give them a reasonable time frame in which to make their move.
The 12 hour rule doesn't always address that neatly. Imagine a scenario in which GFletch usually playes in the 20th hour of the turn because that's what his schedule allows, and Fosse plays his turns whenever he wants to. One day Fosse logs in during the 24th hour after GFletch has played and declares war, landing an enormous stack of units on the coast one tile off GFletch's capital (the transports were unseen, just out of GFletch's line of sight because Fosse is an amazing strategist.
). When Fosse finishes the turn, he presses the red button and a new turn begins. Fosse logs out because 12 hours haven't passed. The next day GFletch has no idea he's at war or that once the 12th hour passes (while he's at work) he's a sitting duck. Fosse logs in at the 18th hour -- well past the rule's proscription -- and razes GFletches capital. Two hours later GFletch logs in to find he's at war and his capital has been razed by an army he didn't even know existed. But no rule has been broken. A player should be able to log on at the same tiem every day and not risk such a situation.
If the rule instead were:
Then Fosse, following the rule, would have waited until the 23rd hour to play if GFletch hadn't logged on by then. At the 23rd hour Fosse would say, "Well, I have to go to bed now so if I don't play my turn, I lose it," and it would be okay. Or GFletch, seeing the email before he left for work, would play in the 10th hour -- throw cannons at Fosse's stack -- and Fosse could go whenever he wanted after that.