The ancient civilization that built complex pyramids in Mexico and is known for its calendar system is the Maya civilization. Flourishing from around 250 A.D. to 900 A.D., the Maya inhabited regions in present-day Mexico, Guatemala, Belize, and parts of Honduras and El Salvador. They are renowned for their impressive architectural achievements, including step pyramids such as those found at Tikal and Chichen Itza. The Maya developed an intricate calendar system based on astronomical observations, which included the Tzolk'in and the Haab' calendars, demonstrating their advanced understanding of time and cyclical patterns in nature. Their cultural and scientific legacy remains influential today.