Converting Julian and Gregorian dates to each other.

ESM only

This package assumes use of the Gregorian calendar and only works correctly for dates after 1858.

Documentation

Install

npm i gregorian-julian.js

pnpm i gregorian-julian.js

yarn add gregorian-julian.js

Calculates the Julian Day (JD) for a given date.

Before diving into the code, let's understand how to calculate the Julian Day (JD) for a given Gregorian date. The following JavaScript function demonstrates this conversion using the gregorian-julian.js library.

import { toJD } from "gregorian-julian.js";
function calculateJulianDay(year, month, day) {
  try {
    const julianDay = toJD(year, month, day);
    console.log(julianDay);
    return julianDay;
  } catch (error) {
    console.error("Error converting date to Julian Day:", error);
  }
}
const julianDay = calculateJulianDay(2022, 10, 15);
// Output: 2459867.5

Converts a Julian Day (JD) to a Gregorian date and time

Now, let's see how we can convert a Julian Day (JD) back to a Gregorian date. The following function uses the gregorian-julian.js library to perform this conversion and prints the result.

import { toGregorian } from "gregorian-julian.js";
function convertJDToGregorian(jd) {
  const result = toGregorian(jd);
  console.log(`Converted Date: ${result.year}-${result.month}-${result.day} ${result.hour}:${result.minute}:${result.second}`);
  return result;
}
const jd = 2459449.5;
const gregorianDate = convertJDToGregorian(jd);
// Output: Converted Date: 2021-8-23 0:0:0

Test and Coverage

Tested with Jest and checked with online converter of aavso, https://www.aavso.org/jd-calculator.

The negative values of JD, before 1 Jan -4712, are considered experimental.

Coverage report