Who invented the calendar?

The modern calendar was created by Pope Gregory XIII in 1582. He introduced the calendar as an adaptation of the Julian version because the drift of Easter, which was tied to the spring equinox, was considered undesirable by the Roman Catholic Church.

Calendars have existed since women marked the lunar cycle on bones to track their menstrual cycles. From those ancient methods of marking time have come a number of calendar systems. The Sumerians, Babylonians, Mayans, Greeks, Romans and Hindus have all had different calendar structures, most of which included 12 months of roughly 30 days each, with periodic leap weeks or leap months to keep the years in step with Earth's rotation.