One of the most brilliantly paradoxical tie-yourself-in-knots time-travel tales is the 1959 short story "All You Zombies" by Robert A. Heinlein. It would be a sin to even hint at the punch-line concept, so all I can do is mention a few of the more quotable bits, like the one near the beginning that has stuck in my mind for several decades now:

Temporal agents always notice time and date; we must.

... and at the end of the yarn, some of the silly sayings posted on the wall:

• Never Do Yesterday What Should Be Done Tomorrow.
• If at Last You Do Succeed, Never Try Again.
• A Stitch in Time Saves Nine Billion.
• A Paradox May Be Paradoctored.
• It Is Earlier When You Think.
• Ancestors Are Just People.
• Even Jove Nods.

I note, belatedly, that Heinlein arranged his aphorisms in order of decreasing length; maybe he (or his editor) was as obsessive about that sort of thing as I am ...

(the full text of the story is posted at [1]; cf. Languages for Smart People (2008-03-12), Aaron's Rod (2009-01-17), ...) - ^z - 2009-03-28