# Calendar Systems – Julian Day Calculator

> Calendar Systems – Julian Day Calculator is a paid API for AI agents from calendar-systems.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-29).

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.

## Facts

- Endpoint: GET https://calendar-systems.x402tookit.com/julian-day?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/calendar-systems-julian-day-calculator-e4f2a0b3
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_nOgOts3zMARIZ9ReWw77L

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability calendar-systems-julian-day-calculator-e4f2a0b3
```

Example prompt: What is the Julian Day and Modified Julian Day for January 1, 2000 at 12:00 UTC in the Gregorian calendar?

## When to prefer this

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.

## Known failure modes

- 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

## How this service works

Julian Day, modified JD and Julian Day Number for a date and UTC time (Gregorian or Julian calendar), or JD back to a date.

## Output

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.

## Request schema (JSON Schema)

```json
{
 "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": {
       "
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/calendar-systems-julian-day-calculator-e4f2a0b3/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from calendar-systems.x402tookit.com](https://www.zero.xyz/host/calendar-systems.x402tookit.com/llms.txt)