Time as in December 21st, 2012 is a manmade construct and has no effect on natural time. For example, if the world was supposed to end at midnight of December 31st 1999, well, December 31, 1999 all ready happened in Sydney, Australia. So since we can safely assume that the world won't end in one hour increments over the course of a whole day, then it's safe to say that manmade time will end when mankind ends.
Natural time, however, is not so predictable. One thing coming before another was the case long before we came around. Something has to happen before something else does.
Human beings die. All multicelluar organisms eventually die. That has always been the case in nature and it will be the case in the future.