When you enter the United States, you may be issued an Arrival/Departure Record (I-94) or a stamp that will be placed in your passport which will include the date by which you must leave the United States.
Note: The I-94 and I-94W process has been automated, and air and sea travelers no longer need to complete the paper Form I-94 or I-94W. A stamp will be place in their passport.
If you are not issued an I-94, but would like to retreive your own I-94 information, you can access it online.
If you are travelling to Canada by land through the United States, it is important that you hand in the I-94 form to the U.S. border officials when you leave the United States. This is your record that you have departed the United States. If you received an electronic I-94 upon arrival by air or sea but depart by land, your departure may not be recorded accurately. For more information visit the U.S. Customs and Border Protection website.
For more information please visit the official website of the Department of Homeland Security.
Instructions on how to record your departure from the United States after you have returned to Canada is available through U.S. Customs and Border Protection.