How authentication forms cookie will be tracked to expiration?

For my application we have not mentioned any explicit timeout and forms authentication cookie is set by default with expiration time 30 min and Sliding Expiration true. I came to know that the sliding expiration will reset the expiration time after 15 min.

In the cookie the only information I see is encrypted text 开发者_StackOverflow中文版and expiry time is not mentioned explicitly. Then How application tracks this cookie to expiration. Will the encrypted text gets updated after 15 min or is this tracking happening on server side for a particular session?