From the I-94 expiration date.

来源: 于 2013-05-16 09:25:50 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

Don't forget to submit the I-94 to the airlines before you leave US. Also keep the airplane boarding card and the air ticket, just in case you need extra proof for your leaving.