The visa expiration date is the date until which you are eligible to travel to a US port-of-entry. If you are permitted to enter the U.S. by an immigration officer at the port-of-entry, the duration for which you are allowed to stay inside the U.S. is known as the authorized period of stay.
A lot of people are unaware of the difference between the visa expiration date and the date of expiration of their authorized period of stay. To travel to the U.S., you must hold a passport and a valid U.S visa. You need to get the right type of visa based on the purpose of your trip to the U.S.
With a visa, you can travel to a U.S. port-of-entry. Immigration officers at the port-of-entry will decide whether to permit you to enter the country and if permitted, for how long you may stay in the U.S. A port-of-entry is an airport or a seaport or a land border crossing in the United States.
Visa expiration date
The date on which your U.S. visa will expire will be printed on your visa. The time between the date on which your visa was issued and the expiry date of the visa is the validity period of your visa. The expiration date printed on the visa is not the same as the time you are permitted to stay in the U.S. For example, if you hold a visa that is valid for ten years, you may travel back and forth to the U.S. during those ten years. It does not mean that you can stay here for ten years.
Length of stay in the U.S.
Once you reach a U.S. port-of-entry, an immigration officer will determine whether to permit you to enter the country and the period of time you are admitted for. When admitting you, the admission stamp or the paper Form I-94, Arrival/Departure Record, will have either a date or D/S (duration of status) stamped on it. If it is a date, then that is the date on which you must leave the U.S. D/S means you can stay in the U.S. for the duration of study/exchange program/employment.
You must not overstay your authorized period of stay. If you stay here beyond the authorized time, you will fall out of status and your visa will be canceled. If you need to stay in the U.S. for longer, you need to apply for an extension of your stay.
Extend your stay
To extend your stay, Form I-539, Application To Extend/Change Nonimmigrant Status must be filed well in advance. At the time of filing for an extension, you need to show that you have not exceeded your authorized stay.
You may also use Form I-539 in case you want to change your visa category. For example, if you first entered the country on a B-2 tourist visa and if you wish to study in the U.S., you must file Form I-539 to change your nonimmigrant status to a student visa before the expiration of your stay.