As a search of this board would show, it's not a new rule. Though it has been tweaked (to be more flexible) since it was originally put into place shortly after the forum began back in '02. As members who were active back then know, it used to limit every member to no more than 10 posts per day every day. However, you'll note that it now says the "average is limited," meaning that members can indeed post more than 10 posts over the course of many days. However, once a given member's period of extended posting over 10 posts per day takes place, the system checks to see if that member's average has gone above 10. If so, that member is limited to 10 posts per day that they post until the average posting rate falls below 10. And as for it being a bit constraining, I doubt many of your fellow members might agree because you were the first person to activate the rule since '02 - and you are but one of only two members to have ever received the error in that intervening time. Also, the forum's top poster (BuzzH) had managed to reach DS God status faster than any other member has ever done it, she currently has made 3093 posts, and her average posting rate per day that she posts is only 7.52 posts - yet she has never once activated the rule. I'm sorry that you feel suddenly constrained - it is certainly not the intent of the admins/mods to stifle anyone - but we do feel that the rule is necessary for the reasons that are laid out in the error that is received should a member activate it.
As for why the system would have told you at 05:31:12 AM ET that you had "2 hours, 4 minutes and 48 seconds" left and then it told you at 09:59:58 AM ET that you had "2 hours, 41 minutes and 52 seconds," I don't honestly know. But I will definitely look into it...