When visiting the United States, the period of stay in the country is set by the Customs and Border Protection officer at the time of entry into the country. Such a statement was made by the U.S. Embassy in Armenia.

According to the Embassy, this period is not related to the validity of the visa. 

The period of stay in the U.S. can be clarified at any time by opening Form I-94 (field՝ admit until date) at i94.cbp.dhs.gov.