In a mixing bowl beat the cream cheese until smooth.
Mix in sweetened condensed milk, key lime juice, and the zest. Blend until well combined.
Fold in whipped topping until light and fluffy
Add a layer of graham crackers to a 9x13 baking dish (breaking some small pieces to completely cover the bottom).
Spread a layer of key lime filling over the graham crackers. Repeat layers until pan is full (3 layers) Finish with filling on top.(note- you will have a few graham crackers left- use to crush for topping). Cover and refrigerate overnight (at least 6-8 hours).
When ready to serve add some crushed graham crackers on top along with some additional lime zest. Slice, serve, and top with whipped cream of desired.