Here's a synopsis from some random dude on IMDB:

Set in medieval Persia, the story of an adventurous prince who teams up with a rival princess to stop an angry ruler from unleashing a sandstorm that could destroy the world. Which is why after the prince was tricked by a dying Vizier to unleash the Sands of Time that turns out to destroy a kingdom and transforms its populace into ferocious demons. In his effort to save his own kingdom and redeem his fatal mistake, it's up to the prince and the princess to return the sands to the hourglass by using the Dagger of Time, which also gives him a limited control over the flow of time.

It's produced by Jerry Bruckheimer and also stars Gemma Arterton and Ben Kingsley.  Gyllenhaal doesn't look right as a Persian prince to me, but I don't plan on seeing this so my opinion really doesn't matter in the least.  Team him up with a monkey in a hat and vest and I'll be game.  Prince of Persia hits theaters May 28th.