Time can be circular or curved in the original 26-D(imensional) string theory of closed strings- only bosons, no matter(mass)- because there are two timelike dimensions in that theory. You can find that theory in the first volume of the string theory books by Green, Schwarz and Witten.

It is also true that my clock starts at zero every twelve hours. But it's an open string that ends at twelve, not zero. It's all a matter of perspective.

yanniru