Fourmilab Switzerland
John Walker: C Ship: The Dilation of Time
A good mathematical account of what happens to time as you approach c: the speed of light. Also provides a rather a fascinating video (fly through the Lattice) - don't miss it. Take some time and explore this site.
University of Virginia
University Virginia/time Dilation: A Worked Example
Here's a solved problem in time dilation. It's still not easy to follow, but it's doable. The math is not difficult, but the mathematical thinking is demanding.