{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"julian-date"},"result":{"entity_type":"calculator","id":"julian-date","calculator_id":"julian-date","canonical_url":"https://tttkmbb.com/time/julian-date","name":"Julian Date Calculator","title":"Julian Date Calculator – Julian Day Number, JD, Modified Julian Date and Days Since J2000 from a Calendar Date","category":"time","category_name":"Time & Geography","tool_name":"convert_julian_date","featured_mcp_tool":false,"description":"Converts a Gregorian calendar date and time (in UTC or an IANA zone) to the astronomical Julian Day Number, Julian Date (JD), Modified Julian Date (MJD), Truncated Julian Date, days since the J2000.0 epoch and Julian centuries, plus day of year and ISO week.","use_when":"You need JD/MJD values for astronomy, satellite or ephemeris work, or want to know the day count between dates via Julian Day Numbers.","do_not_use_when":"You mean the 'Julian date' used in food packaging or the military (a day-of-year number such as 2026-266; see day-of-week for day of year) or Unix epoch seconds (use unix-timestamp).","inputs":[{"name":"date","label":"Date","type":"date","required":false,"default":"today","description":"Gregorian calendar date (YYYY-MM-DD); defaults to the current UTC date.","example":"2000-01-01"},{"name":"time","label":"Time","type":"string","required":false,"default":"00:00","description":"Time of day, 24-hour HH:MM (seconds optional), in time_zone.","example":"12:00"},{"name":"time_zone","label":"Time zone","type":"string","required":false,"default":"UTC","description":"IANA zone of the given date and time; JD is always expressed in Universal Time.","example":"UTC"}],"outputs":[{"name":"julian_day_number","label":"Julian Day Number (JDN)","type":"integer","decimals":4,"description":"Integer day count of the UTC calendar date (the JD at 12:00 UT that day); JDN 0 = 1 January 4713 BC (Julian)."},{"name":"julian_date","label":"Julian Date (JD)","type":"number","unit":"days","decimals":5,"description":"Days since noon UT on 1 January 4713 BC, with the fraction of the day since noon."},{"name":"modified_julian_date","label":"Modified Julian Date (MJD)","type":"number","unit":"days","decimals":5,"description":"JD − 2,400,000.5 (starts at midnight UT on 17 November 1858)."},{"name":"truncated_julian_date","label":"Truncated Julian Date (TJD)","type":"integer","decimals":4,"description":"⌊JD − 2,440,000.5⌋, NASA convention counting from 24 May 1968."},{"name":"days_since_j2000","label":"Days since J2000.0","type":"number","unit":"days","decimals":5,"description":"JD − 2,451,545.0 (J2000.0 = 2000-01-01 12:00 TT, taken here as UT)."},{"name":"julian_century","label":"Julian centuries since J2000.0","type":"number","decimals":8,"description":"(JD − 2,451,545) / 36,525, the T used in astronomical formulas."},{"name":"utc_datetime","label":"UTC date-time","type":"string","decimals":4,"description":"The instant in UTC (ISO 8601)."},{"name":"day_of_week","label":"Weekday (UTC)","type":"string","decimals":4,"description":"Day of the week of the UTC date."},{"name":"day_of_year","label":"Day of year (UTC)","type":"integer","decimals":4,"description":"Ordinal day of the UTC date, 1 January = 1."},{"name":"iso_week_date","label":"ISO week date","type":"string","decimals":4,"description":"UTC date in ISO 8601 week notation YYYY-Www-D."}],"input_schema":{"type":"object","properties":{"date":{"description":"Gregorian calendar date (YYYY-MM-DD); defaults to the current UTC date.","type":"string","format":"date","default":"today","examples":["2000-01-01"]},"time":{"description":"Time of day, 24-hour HH:MM (seconds optional), in time_zone.","type":"string","default":"00:00","examples":["12:00"]},"time_zone":{"description":"IANA zone of the given date and time; JD is always expressed in Universal Time.","type":"string","default":"UTC","examples":["UTC"]}},"additionalProperties":false},"output_schema":{"type":"object","properties":{"julian_day_number":{"description":"Integer day count of the UTC calendar date (the JD at 12:00 UT that day); JDN 0 = 1 January 4713 BC (Julian).","type":"integer"},"julian_date":{"description":"Days since noon UT on 1 January 4713 BC, with the fraction of the day since noon. Unit: days.","type":"number","x-unit":"days"},"modified_julian_date":{"description":"JD − 2,400,000.5 (starts at midnight UT on 17 November 1858). Unit: days.","type":"number","x-unit":"days"},"truncated_julian_date":{"description":"⌊JD − 2,440,000.5⌋, NASA convention counting from 24 May 1968.","type":"integer"},"days_since_j2000":{"description":"JD − 2,451,545.0 (J2000.0 = 2000-01-01 12:00 TT, taken here as UT). Unit: days.","type":"number","x-unit":"days"},"julian_century":{"description":"(JD − 2,451,545) / 36,525, the T used in astronomical formulas.","type":"number"},"utc_datetime":{"description":"The instant in UTC (ISO 8601).","type":"string"},"day_of_week":{"description":"Day of the week of the UTC date.","type":"string"},"day_of_year":{"description":"Ordinal day of the UTC date, 1 January = 1.","type":"integer"},"iso_week_date":{"description":"UTC date in ISO 8601 week notation YYYY-Www-D.","type":"string"}}},"formula":"a = ⌊(14 − month)/12⌋, y = year + 4800 − a, m = month + 12a − 3; JDN = day + ⌊(153m + 2)/5⌋ + 365y + ⌊y/4⌋ − ⌊y/100⌋ + ⌊y/400⌋ − 32045; JD = JDN − 0.5 + (hours_UT / 24); MJD = JD − 2400000.5","method":"Proleptic Gregorian calendar; times are converted to UT via the tz database before the day fraction is added. Leap seconds and the ~69 s difference between UT and Terrestrial Time are ignored (relevant only for sub-minute ephemeris work).","sources":[{"name":"USNO Astronomical Applications – Julian Date Converter","url":"https://aa.usno.navy.mil/data/JulianDate","type":"government","retrieved_at":"2026-09-24"},{"name":"Julian day – JDN algorithms and epochs (Wikipedia)","url":"https://en.wikipedia.org/wiki/Julian_day","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"daily","max_age_seconds":86400,"note":"Depends on the current date unless the date is supplied."},"examples":[{"name":"J2000.0 epoch: 2000-01-01 12:00 UTC","inputs":{"date":"2000-01-01","time":"12:00","time_zone":"UTC"},"expected":{"julian_day_number":2451545,"julian_date":2451545,"modified_julian_date":51544.5,"truncated_julian_date":11544,"days_since_j2000":0,"julian_century":0,"day_of_week":"Saturday","day_of_year":1,"iso_week_date":"1999-W52-6"},"url":"https://tttkmbb.com/api/v1/calculate/julian-date?date=2000-01-01&time=12%3A00&time_zone=UTC"},{"name":"MJD epoch: 1858-11-17 00:00 UTC","inputs":{"date":"1858-11-17","time":"00:00"},"expected":{"julian_date":2400000.5,"modified_julian_date":0,"julian_day_number":2400001,"day_of_week":"Wednesday"},"url":"https://tttkmbb.com/api/v1/calculate/julian-date?date=1858-11-17&time=00%3A00"}],"faq":[{"q":"Why does the Julian Date have a .5 at midnight?","a":"Julian days begin at noon UT, so midnight falls halfway through a Julian day: 2000-01-01 00:00 UT is JD 2451544.5 and 12:00 UT is JD 2451545.0. MJD shifts the origin to midnight to avoid this."},{"q":"Is the Julian Date related to the Julian calendar?","a":"Only historically; the count was proposed by Joseph Scaliger in 1583 and named after his father Julius. The calendar date entered here is Gregorian."}],"tags":["julian date","julian day number","modified julian date","mjd","j2000","astronomical date"],"related":[{"calculator_id":"unix-timestamp","reason":"Unix epoch value of the same instant."},{"calculator_id":"day-of-week","reason":"Weekday and ordinal day of year of a date."},{"calculator_id":"date-difference","reason":"Days between two dates (equal to the JDN difference)."}],"links":{"html":"https://tttkmbb.com/time/julian-date","markdown":"https://tttkmbb.com/time/julian-date.md","json":"https://tttkmbb.com/time/julian-date.json","api":"https://tttkmbb.com/api/v1/calculate/julian-date","schema":"https://tttkmbb.com/api/v1/calculators/julian-date","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T01:45:33Z"}