In Deathly Hallows, why did Snape tell Voldemort the escape plan?

Snape knew because he's still inside the order, in other words: Lupin and Mr. Weasly knew that Dumbledore had trusted Snape for a reason. Snape already knew the correct date and noticed the fake date. He told Voldemort because he needed to make sure Voldemort trusted him until the exact moment Harry needed him.