Throughout the centuries, human societies developed different systems to organize the passage of time, according to their beliefs, productive activities, and forms of collective organization. Early human communities needed to anticipate key natural cycles, such as seasons and lunar phases. One of the oldest systems, estimated to be 10,000 years old, was discovered in Scotland. In the Roman world, the calendar went through multiple transformations. Its earliest versions lacked months for winter and structured the year around ten periods that began in March, a moment associated with the resumption of agricultural activity and the worship of the god Mars. Over time, this scheme showed discrepancies with the actual course of the seasons. To correct them, in the 7th century BC, King Numa Pompilius incorporated two new months: January and February. However, the use of lunar references prevented achieving a precise and stable correspondence with the solar year. The most influential reform took place in 45 BC, when Julius Caesar promoted a calendar based on the movement of the sun. This system, known as the Julian calendar, established an annual duration of 365 days and set the beginning of the year in January, alluding to Janus, a deity associated with beginnings, doors, and passages. Despite its spread, the date of New Year's continued to be diverse for centuries. In several Christian territories, the beginning of the year was celebrated on March 25, a day linked to the Annunciation and the symbolic beginning of the life of Jesus Christ. The calendar that governs most of the planet today—the Gregorian—is the result of a long succession of adjustments and consensus, and not an immutable convention from its origins. The final standardization arrived only in 1582, with the promulgation of the Gregorian calendar by Pope Gregory XIII, which introduced the leap year system to correct the accumulated discrepancies.
