The calendar that is in general use today is the Gregorian calendar; it dates to 1582, when Pope Gregory XIII (1502–1585) asked for a revision of the Julian calendar. That calendar is named for its initiator, Julius Caesar (100–44 B.C.), who in about 46 B.C. commissioned the astronomer Sosigenes of Alexandria to develop a universal solar calendar to be used throughout the Roman Empire (as Roman armies conquered more and more territory, the empire included many peoples and differing calendars, including the lunar-based Roman calendar). The Julian calendar consisted of a year of 365 days, with one day added every fourth year (leap year, when the year is divisible by four) to compensate for the fact that the solar year is really 365.25 days. It had 12 months, each of 30 or 31 days except February, which had 28, and the new year began on January 1. The Gregorian calendar retained these features but revised the Julian to bring the Christian celebration of Easter in alignment with the vernal equinox (first day of spring). It also dropped leap years for any century year not divisible by 400—an effort to keep the solar calendar in line with the seasons: For example, 1900, though divisible by four, was not a leap year since it was a centenary year not divisible by 400; the year 2000, divisible by 400, was a centenary leap year.

Other calendars remain in use in the world today, including the lunar Babylonian, Chinese, and Muslim calendars; the Jewish calendar, which is a combination of solar and lunar; and the solar Coptic, Japanese, and Hindu calendars. Secular calendars include the Julian Day, used by astronomers; and the perpetual calendar, which gives the days of the week for the Julian and the Gregorian calendar, and therefore is used by historians and other scholars to reconcile world events along a single timeline.