For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
If we do have an appointed time of death and can't do anything to change that time then that would seem to indicate that we have no ability to make any choice or have free will.
I've always been told that we can go before our time. If we (Christians) live as a bad testimony, He will take us home before our time. It's up to us to live properly and for Jesus as testimonies for the lost, but if we cannot do this, God will take us home.