The United States uses the Gregorian calendar, which is a solar calendar.

both. the Aztecs had 2 calendars.

Lunar calendar

solar

A solar calendar is a type of calendar that is based on the position of the Earth in relation to the sun. This means that the calendar is organized around the length of the solar year, which is the time it takes for the Earth to complete one orbit around the sun. Solar calendars are used to keep track of the seasons and to organize events and appointments throughout the year. Some examples of solar calendars include the Gregorian Calendar, the Julian Calendar, and the Hindu Calendar. The solar calendar is the most widely used calendar system in the world and is the standard for civil and commercial purposes.