How they did it

The pie clock involves a little sleight of hand. Here are the pieces that make up the clock:

It's a bit like a sandwich. We start by drawing the blue background. Then, we layer on the two yellow half-moon pieces. To make the clock look pretty, we apply the lovely, shiny gloss picture in front of all these pieces.

We rotate the right half-moon piece halfway around the circle to make the slice of yellow time appear to grow smaller and smaller.

At the halfway point, we slap a half-moon section of the blue background on top of everything, on the right side of the clock. We're all done with the right half-circle, so we don't draw it.

Now, we rotate the left half-moon piece, and it disappears behind the blocker graphic, creating the illusion that the rest of the clock is depleting.

When time is up, we slap the red clock graphic in front of everything.

A little flourish of the wrist, a puff of smoke, and the audience doesn't suspect a thing!