{"uid":"cap_nOgOts3zMARIZ9ReWw77L","slug":"calendar-systems-julian-day-calculator-e4f2a0b3","name":"Calendar Systems – Julian Day Calculator","description":"Julian Day, modified JD and Julian Day Number for a date and UTC time (Gregorian or Julian calendar), or JD back to a date.","url":"https://calendar-systems.x402tookit.com/julian-day?utm_source=zero.xyz","method":"GET","headers":{},"bodySchema":{"type":"object","$schema":"https://json-schema.org/draft/2020-12/schema","required":["input"],"properties":{"input":{"type":"object","required":["type","method"],"properties":{"type":{"type":"string","const":"http"},"method":{"enum":["GET"],"type":"string"},"queryParams":{"type":"object","required":[],"properties":{"jd":{"type":"number","description":"A Julian Day to convert to a date instead (0 to 5373484)"},"date":{"type":"string","description":"YYYY-MM-DD, year -4712 to 9999 (astronomical numbering: year 0 = 1 BCE)"},"time":{"type":"string","description":"UTC time HH:MM or HH:MM:SS, default 00:00:00"},"calendar":{"enum":["gregorian","julian"],"type":"string","description":"gregorian (default, proleptic) or julian: how the date is written"}}}},"additionalProperties":false},"output":{"type":"object","required":["type"],"properties":{"type":{"type":"string"},"example":{"type":"object","properties":{"asOf":{"type":["null","string"]},"source":{"type":["null","string"]},"rataDie":{"type":["null","number"]},"reasons":{"type":["array","null"],"items":{"type":"string"}},"summary":{"type":["null","string"]},"verdict":{"type":["null","string"]},"weekday":{"type":["null","string"]},"degraded":{"type":["boolean","null"]},"julianDay":{"type":["null","number"]},"disclaimer":{"type":["null","string"]},"notChecked":{"type":["array","null"],"items":{"type":"string"}},"j2000Offset":{"type":["null","number"]},"calendarUsed":{"type":["null","string"]},"gregorianDate":{"type":["null","string"]},"julianDayNumber":{"type":["null","number"]},"modifiedJulianDay":{"type":["null","number"]},"julianCalendarDate":{"type":["null","string"]}}}}}}},"responseSchema":null,"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.02","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.02/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.02","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_y847g-p54HLEpJqi6T96M","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.02","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"Converts a Gregorian or Julian calendar date and UTC time to Julian Day (JD), Modified Julian Day (MJD), and Julian Day Number (JDN), or converts a JD number back to a calendar date.","exampleAgentPrompt":"What is the Julian Day and Modified Julian Day for January 1, 2000 at 12:00 UTC in the Gregorian calendar?","exampleUseCases":[{"title":"Astronomical epoch reference calculation","prompt":"I need the Julian Day number and J2000 offset for March 20, 2024 at 03:06 UTC in the Gregorian calendar — it's for an astronomical observation log."},{"title":"Historical date back-conversion from JD","prompt":"Can you tell me what calendar date corresponds to Julian Day 2299160? Give me both the Gregorian and Julian calendar dates."},{"title":"Julian calendar date to JD for medieval research","prompt":"I'm researching a medieval document dated October 4, 1582 in the Julian calendar — what is the Julian Day number and Modified Julian Day for that date?"}],"resultDescription":"Returns Julian Day (fractional), Modified Julian Day, Julian Day Number (integer), weekday name, Rata Die, J2000 offset, the input date expressed in both Gregorian and Julian calendar forms, and the calendar used for the calculation. May also include a summary, disclaimer, and degraded-status flag.","failureModes":["Date out of range (year must be between -4712 and 9999) returns error or null values","JD value out of range (must be 0 to 5373484) when converting back to a date","Invalid time format for the time parameter returns an error","Missing required calendar or date/jd parameter returns validation error","Non-numeric or malformed date string causes parse failure"],"whenToPreferThis":"Use this endpoint when you need precise Julian Day, Modified Julian Day, or Julian Day Number values for astronomical, historical, or scientific calculations involving specific dates and times. It is particularly valuable when working with dates in both Gregorian and Julian calendar systems, including proleptic Gregorian dates before 1582 and historical Julian calendar dates. Prefer this over general date-conversion endpoints when JD or MJD precision is specifically required.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-10-29T13:12:36.557Z","isFirstParty":false,"canonicalSlug":"calendar-systems-julian-day-calculator-e4f2a0b3"}