Warning, time traveler: if you're reading this, it's already happened. The universe has unfolded in a way that makes this message possible. Don't try to change the course of events; you might just end up where you started.
Multiverse Problems: Because you thought this was just one timeline?, Predestination Paradox: Because you were always going to end up here?
ChronoLogic: A Calculator for the Chronologically Challenged