Baking Time for a Smaller Cake Pan

I'm baking a sheet cake for my sister's birthday and I don't have a 13x9 inch pan. I do have a 9x9x2 inch pan. If I use the 9x9 inch pan, how long do I have to cook it for? The usual time for the 13x9 inch is 22 minutes. Help! A fast reply would be appreciated as I am rapidly running out of time.

By Saumensch from Zim