Well, on DS one day could often stretch into a week or more, so perhaps there's no telling how long 120 seconds might actually last in the DS world.
But seriously, remember that if anyone happens to try to post something before the initial 120 seconds completely elapses, the system begins counting another full 120 seconds all over again. Meaning that if, say, someone's last post was at 08:00:00, they should really wait until 08:02:01 to post again. But if they tried to submit another post after only 119 seconds had elapsed at 08:01:59, the 120 second count would start all over again, meaning they should really wait until after the second full 120 seconds had elapsed by 08:04:00 to post again. Repeated attempts to try to post before the full 120 seconds actually elapses could stretch the wait between posts out for who knows how long?