The history of calendars, that is, of people creating and using methods for keeping track of days and larger divisions of time, covers a practice with ancient roots. Archeologists have reconstructed methods of timekeeping that go back to prehistoric times at least as old as the Neolithic. The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Calendars are explicit schemes used for timekeeping. The first historically attested and formulized calendars date to the Bronze Age, dependent on the development of writing in the ancient Near East. The Sumerian calendar was the earliest, followed by the Egyptian, Assyrian and Elamite calendars. A larger number of calendar systems of the ancient Near East appear in the Iron Age archaeological record, based on the Assyrian and Babylonian calendars. This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew calendar. Calendars in antiquity were usually lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar. Nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan 10-month solar year.The Roman calendar was reformed by Julius Caesar in 45 BC. The Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation. In the 11th century in Persia, a calendar reform led by Khayyam was announced in 1079, when the length of the year was measured as 365.24219858156 days. Given that the length of the year is changing in the sixth decimal place over a person's lifetime, this is outstandingly accurate. For comparison the length of the year at the end of the 19th century was 365.242196 days, while today it is 365.242190 days.The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582, and is today in worldwide use as the "de facto" calendar for secular purposes.