America uses the Gregorian calendar. The Gregorian calendar was introduced by Pope Gregory XIII in October 1582 to correct the inaccuracies of the Julian calendar. The Julian calendar, introduced by Julius Caesar in 45 BCE, had a slight miscalculation in the length of the year, causing the calendar to drift out of alignment with the solar year over centuries.
Most of the world, including America, adopted the Gregorian calendar to ensure consistency in timekeeping. The transition from the Julian to the Gregorian calendar in America occurred when the British Empire, which included the American colonies, adopted the Gregorian calendar in 1752. This change involved skipping 11 days to realign the calendar with the solar year.
Today, the Gregorian calendar is the standard civil calendar used in America and most other countries around the world. It is the calendar system used for everyday purposes, including business, government, and personal scheduling.