O site que você cita é uma fonte "semi-oficial", administrada por um contratante do governo (graças a Michael Hampton para apontar isso). Fontes oficiais concordam; veja o link :
Admission to the United States and your Duration of Stay
Upon arriving at a port of entry, the CBP official will determine the length of your visit.
On the admission stamp or paper Form I-94, the U.S. immigration inspector records either an admitted-until date or "D/S" (duration of status). If your admission stamp or paper Form I-94 contains a specific date, then that is the date by which you must leave the United State. If you have D/S on your admission stamp or paper Form I-94, you may remain in the United States as long as you continue your course of studies, remain in your exchange program, or qualifying employment. The admitted-until date or D/S notation, shown on your admission stamp or paper Form I-94 is the official record of your authorized length of stay in the United States. You cannot use the visa expiration date in determining or referring to your permitted length of stay in the United States.
Carefully review information about international visitor admission on the CBP Website.
Se o seu visto é um visto B-2 (ou B-1 / B-2), você normalmente receberá seis meses de entrada, então você deve estar bem.
Se você tiver outro tipo de visto, o seu período de admissão pode ser diferente e pode depender da finalidade da sua viagem.