Julian Day and Sexagenary Day

Last revised: 2026-10-18

This page turns a Gregorian date and time into the Julian Day (JD), the Modified Julian Day (MJD) and the Julian Day Number (JDN), and gives the day of year, the weekday and the sexagenary day of that date. It also runs the other way: from a Julian Day back to a Gregorian date and time.

The Julian Day is a continuous count of days. It starts at noon Universal Time on 1 January 4713 BCE; each day is one whole number, and the time of day is the fraction. Month lengths, leap years and calendar reforms leave it untouched, so the interval between two moments is a single subtraction. The solar-term times, the new-moon and full-moon times and the sexagenary day count on this site are all aligned to it underneath.

The sexagenary day is fixed by the Julian Day Number plus 49, taken modulo 60, with 1 October 1949 as the jiazi day. This is the same rule as on the converter page and the lunar-mansion sexagenary-day page of this site, and it agrees with the continuous sexagenary day count laid down in the Chinese national standard GB/T 33661-2017, The Calculation and Promulgation of the Chinese Calendar, clause 6.3.2 and Annex C.

Gregorian date

Time (Beijing Time)

Reverse: Julian Day to Gregorian date

Conventions

All Gregorian dates use the Gregorian calendar, the calendar in use from 15 October 1582 onward. The tool does not extend to dates before that day.

The date and time you enter are Beijing Time, the zone time of 120° E (UTC+8). The Julian Day is reckoned in Universal Time (UT), and the conversion has already subtracted eight hours. UT and Coordinated Universal Time (UTC) differ by less than one second; the tool does not distinguish them.

The Julian Day Number (JDN) is the integer day number at noon of that date. JD = JDN + (UT time − 12) / 24. MJD = JD − 2400000.5, and its day begins at midnight.

The sexagenary day follows the Gregorian day and turns over at midnight. The day pillar in the Four Pillars turns over at the zi hour instead, which is a different rule; the converter page on this site sets out the difference.

The lunar input on this page is further limited by the ephemeris and covers 1901 to 2052 only. Outside that span the Gregorian conversion works as usual; lunar dates are neither given nor extrapolated.

Limits

Dates run from 15 October 1582 to 31 December 2200. The lower bound is the day the Gregorian calendar came into use; the upper bound is the tool’s own computational limit. Nothing outside the range is converted.

The sexagenary day is a purely arithmetic continuous count. Whether it agrees with the sexagenary days written in historical sources has to be checked against those sources separately; the Historical Records against Computed Values page of this site holds such comparisons.

Sources

Purple Mountain Observatory, Chinese Academy of Sciences, The Chinese Astronomical Almanac (中国天文年历), Appendix Table 1, “Julian Day”; and the annual almanac data listed on the observatory’s Almanac Query page.

Ephemeris Computation Office, National Astronomical Observatory of Japan, Calendar and Ephemeris (暦象年表), “Fundamental Values”: ユリウス日, ユリウス日→年月日, 通日・曜日・干支.

National standard GB/T 33661-2017, The Calculation and Promulgation of the Chinese Calendar.