Understanding Your Jewish Birthday: A Unique Calendar Journey
The concept of a birthday is universal, a day to celebrate one's entry into the world. However, for those of Jewish heritage, there are often two birthdays to acknowledge: the secular (Gregorian) birthday and the Jewish (Hebrew) birthday. While the Gregorian calendar is widely used for daily life, the Hebrew calendar holds deep religious and cultural significance, dictating the timing of holidays, lifecycle events, and, of course, birthdays.
The Hebrew Calendar: A Lunar-Solar System
Unlike the purely solar Gregorian calendar, the Hebrew calendar is a lunisolar calendar. This means it tracks both the cycles of the moon (for months) and the sun (for years). A standard Hebrew year has 12 lunar months, but to keep it aligned with the solar year and ensure holidays like Passover fall in the spring, a leap month (Adar I) is added seven times in a 19-year cycle. This intricate system ensures that Jewish holidays and birthdays maintain their seasonal relevance.
Lunar Months: Each month begins with the appearance of the new moon. Months typically alternate between 29 and 30 days.
Solar Years: The addition of a leap month (Adar I) in specific years (3rd, 6th, 8th, 11th, 14th, 17th, and 19th years of the 19-year cycle) ensures the calendar stays roughly aligned with the solar year.
Rosh Chodesh: The beginning of each new month is a minor holiday called Rosh Chodesh.
Rosh Hashanah: The Jewish New Year, 1 Tishrei, marks the beginning of a new Hebrew year.
Why is a Jewish Birthday Important?
For many, a Jewish birthday is more than just another date; it's a day of spiritual significance. Tradition teaches that on one's Jewish birthday, a person's mazal (fortune or spiritual energy) is particularly strong. It's considered an auspicious time for introspection, prayer, making good resolutions, and giving charity. Some customs include:
Reciting extra Psalms.
Studying Torah.
Giving extra charity (tzedakah).
Making a personal commitment to improve in a specific area.
Having a festive meal with family and friends.
For boys, the Bar Mitzvah (at age 13) is celebrated on their Jewish birthday, and for girls, the Bat Mitzvah (at age 12). These milestones mark a transition to religious adulthood according to Jewish law.
How Does the Calculator Work?
Our Jewish Birthday Calculator takes your Gregorian birth date (Day, Month, Year) and performs a series of complex calendrical conversions:
Gregorian to Julian Day Number (JDN): First, your Gregorian birth date is converted into a Julian Day Number, a continuous count of days since January 1, 4713 BCE. This serves as a universal reference point for calendar conversions.
JDN to Hebrew Date: Using the JDN, the calculator then applies the intricate rules of the Hebrew calendar to determine the exact Hebrew year, month, and day of your birth. This involves calculating the precise timing of the new moon (molad) for Tishrei (the first month of the Hebrew year) and applying various postponement rules (dehiyyot) to determine the start of each Hebrew year. It also accounts for the variable lengths of months like Cheshvan and Kislev, and the addition of Adar I in leap years.
Finding the Next Jewish Birthday: Once your Hebrew birth date is established, the calculator determines the current Hebrew year based on today's Gregorian date. It then finds the next occurrence of your Hebrew birth date in the Gregorian calendar, taking into account whether the current Hebrew year is a leap year or a common year, and how this affects months like Adar (Adar I, Adar II, or just Adar). If your Hebrew birthday has already passed in the current Hebrew year, it calculates it for the following Hebrew year.
Calculating Hebrew Age: Your age in Hebrew years is determined by subtracting your Hebrew birth year from the current Hebrew year, with an adjustment if your birthday has not yet occurred in the current Hebrew year.
This calculator simplifies a complex process, allowing you to easily discover your Jewish birth date and anticipate your next Jewish birthday, connecting you to a rich tradition and a unique way of marking time.
Jewish Birthday Calculator
// — Hebrew Calendar Logic (Adapted from Dershowitz & Reingold) —
// Constants for Hebrew calendar calculations
var HEBREW_EPOCH = 347997; // JDN of 1 Tishrei, 1 AM, year 1 (Gregorian Oct 7, 3761 BCE)
var MOLAD_TISHREI_1_1_PARTS = 204; // Molad of Tishrei, year 1, in parts (1 hour = 1080 parts)
var MOLAD_TISHREI_1_1_HOURS = 5; // Molad of Tishrei, year 1, in hours
var MOLAD_TISHREI_1_1_DAYS = 2; // Molad of Tishrei, year 1, in days (Monday = 2, Sunday = 1)
var LUNAR_MONTH_PARTS = 29 * 1080 + 204 + 1; // 29 days, 12 hours, 793 parts (29.530594 days)
// Helper function to calculate the molad of Tishrei for a given Hebrew year
function molad(hebrewYear) {
var monthsElapsed = Math.floor((235 * hebrewYear – 234) / 19); // Number of lunar months since creation
var moladParts = MOLAD_TISHREI_1_1_PARTS + monthsElapsed * LUNAR_MONTH_PARTS;
var moladDay = MOLAD_TISHREI_1_1_DAYS + Math.floor(moladParts / (24 * 1080));
var moladHours = Math.floor((moladParts % (24 * 1080)) / 1080);
var moladRemainingParts = (moladParts % (24 * 1080)) % 1080;
return {
day: moladDay, // Day of the week (1=Sunday, 2=Monday…)
hours: moladHours,
parts: moladRemainingParts
};
}
// Determines if a Hebrew year is a leap year
function isHebrewLeapYear(hebrewYear) {
var b = (hebrewYear * 7 + 1);
var c = Math.floor(b / 19);
var d = b % 19;
return (d == 0 || d == 3 || d == 6 || d == 8 || d == 11 || d == 14 || d == 17);
}
// Calculates the JDN of 1 Tishrei for a given Hebrew year
function hebrewYearStartJDN(hebrewYear) {
var m = molad(hebrewYear);
var moladDayOfWeek = m.day % 7; // 0=Saturday, 1=Sunday, 2=Monday… (JDN day of week)
var JDN = HEBREW_EPOCH + Math.floor(m.day / 7) * 7; // Base JDN for the week of molad
// Apply Dehiyyot (postponements)
// Dehiyyah 1: If molad is on Sunday, Wednesday, or Friday, Rosh Hashanah is postponed to the next day.
// (JDN day of week: 1=Sunday, 4=Wednesday, 6=Friday)
if (moladDayOfWeek == 1 || moladDayOfWeek == 4 || moladDayOfWeek == 6) {
JDN++;
}
// Dehiyyah 2: If molad is after 18 hours (noon)
if (m.hours >= 18) {
JDN++;
}
// Dehiyyah 3: If molad of a common year is on a Tuesday after 9 hours and 204 parts
// (JDN day of week: 3=Tuesday)
if (!isHebrewLeapYear(hebrewYear) && moladDayOfWeek == 3 && (m.hours * 1080 + m.parts) >= (9 * 1080 + 204)) {
JDN += 2; // Postpone to Thursday
}
// Dehiyyah 4: If molad of a year following a leap year is on a Monday after 15 hours and 589 parts
// (JDN day of week: 2=Monday)
if (isHebrewLeapYear(hebrewYear – 1) && moladDayOfWeek == 2 && (m.hours * 1080 + m.parts) >= (15 * 1080 + 589)) {
JDN++;
}
// Re-check Dehiyyah 1 after other postponements
var finalDayOfWeek = (JDN – HEBREW_EPOCH + MOLAD_TISHREI_1_1_DAYS) % 7;
if (finalDayOfWeek == 1 || finalDayOfWeek == 4 || finalDayOfWeek == 6) {
JDN++;
}
return JDN;
}
// Calculates the length of a Hebrew year in days
function hebrewYearLength(hebrewYear) {
var startJDN = hebrewYearStartJDN(hebrewYear);
var nextStartJDN = hebrewYearStartJDN(hebrewYear + 1);
return nextStartJDN – startJDN;
}
// Determines the type of Hebrew year (deficient, regular, complete)
function getHebrewYearType(hebrewYear) {
var len = hebrewYearLength(hebrewYear);
var isLeap = isHebrewLeapYear(hebrewYear);
if (isLeap) {
if (len == 383) return "deficient_leap";
if (len == 384) return "regular_leap";
if (len == 385) return "complete_leap";
} else {
if (len == 353) return "deficient_common";
if (len == 354) return "regular_common";
if (len == 355) return "complete_common";
}
return "unknown"; // Should not happen
}
// Converts a JDN to a Hebrew date (year, month, day)
function jdnToHebrew(jdn) {
var hebrewYear = Math.floor((jdn – HEBREW_EPOCH) / 365.2425) + 1; // Approximation
while (jdn = hebrewYearStartJDN(hebrewYear + 1)) {
hebrewYear++;
}
var yearStartJDN = hebrewYearStartJDN(hebrewYear);
var daysIntoYear = jdn – yearStartJDN;
var isLeap = isHebrewLeapYear(hebrewYear);
var yearType = getHebrewYearType(hebrewYear);
var hebrewMonthNames = ["Tishrei", "Cheshvan", "Kislev", "Tevet", "Shevat", "Adar", "Nisan", "Iyar", "Sivan", "Tammuz", "Av", "Elul"];
var hebrewLeapMonthNames = ["Tishrei", "Cheshvan", "Kislev", "Tevet", "Shevat", "Adar I", "Adar II", "Nisan", "Iyar", "Sivan", "Tammuz", "Av", "Elul"];
var monthLengthsArr = [];
if (isLeap) {
monthLengthsArr = [30, 29, 30, 29, 30, 30, 29, 30, 29, 30, 29, 30, 29]; // Tishrei to Elul, with Adar I & II
if (yearType.includes("deficient")) { // Cheshvan 29, Kislev 29
monthLengthsArr[1] = 29;
monthLengthsArr[2] = 29;
} else if (yearType.includes("complete")) { // Cheshvan 30, Kislev 30
monthLengthsArr[1] = 30;
monthLengthsArr[2] = 30;
} else { // Regular leap year: Cheshvan 29, Kislev 30
monthLengthsArr[1] = 29;
monthLengthsArr[2] = 30;
}
} else {
monthLengthsArr = [30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29]; // Tishrei to Elul
if (yearType.includes("deficient")) { // Cheshvan 29, Kislev 29
monthLengthsArr[1] = 29;
monthLengthsArr[2] = 29;
} else if (yearType.includes("complete")) { // Cheshvan 30, Kislev 30
monthLengthsArr[1] = 30;
monthLengthsArr[2] = 30;
} else { // Regular common year: Cheshvan 29, Kislev 30
monthLengthsArr[1] = 29;
monthLengthsArr[2] = 30;
}
}
var currentMonthIndex = 0;
var currentDay = daysIntoYear + 1;
for (var i = 0; i < monthLengthsArr.length; i++) {
if (currentDay <= monthLengthsArr[i]) {
currentMonthIndex = i;
break;
}
currentDay -= monthLengthsArr[i];
}
return {
year: hebrewYear,
month: (isLeap ? hebrewLeapMonthNames[currentMonthIndex] : hebrewMonthNames[currentMonthIndex]),
day: currentDay,
monthIndex: currentMonthIndex // 0-indexed
};
}
// Converts a Hebrew date to a JDN
function hebrewToJDN(hebrewYear, hebrewMonthName, hebrewDay) {
var yearStartJDN = hebrewYearStartJDN(hebrewYear);
var isLeap = isHebrewLeapYear(hebrewYear);
var yearType = getHebrewYearType(hebrewYear);
var hebrewMonthNames = ["Tishrei", "Cheshvan", "Kislev", "Tevet", "Shevat", "Adar", "Nisan", "Iyar", "Sivan", "Tammuz", "Av", "Elul"];
var hebrewLeapMonthNames = ["Tishrei", "Cheshvan", "Kislev", "Tevet", "Shevat", "Adar I", "Adar II", "Nisan", "Iyar", "Sivan", "Tammuz", "Av", "Elul"];
var monthIndex = -1;
if (isLeap) {
monthIndex = hebrewLeapMonthNames.indexOf(hebrewMonthName);
} else {
monthIndex = hebrewMonthNames.indexOf(hebrewMonthName);
// If it's a common year and the month is Adar I or Adar II, it's an error or needs mapping to Adar
if (hebrewMonthName === "Adar I" || hebrewMonthName === "Adar II") {
monthIndex = hebrewMonthNames.indexOf("Adar"); // Map Adar I/II to Adar for common year
}
}
if (monthIndex === -1) {
return -1; // Invalid month name
}
var monthLengthsArr = [];
if (isLeap) {
monthLengthsArr = [30, 29, 30, 29, 30, 30, 29, 30, 29, 30, 29, 30, 29]; // Tishrei to Elul, with Adar I & II
if (yearType.includes("deficient")) {
monthLengthsArr[1] = 29; monthLengthsArr[2] = 29;
} else if (yearType.includes("complete")) {
monthLengthsArr[1] = 30; monthLengthsArr[2] = 30;
} else { // Regular leap
monthLengthsArr[1] = 29; monthLengthsArr[2] = 30;
}
} else {
monthLengthsArr = [30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29]; // Tishrei to Elul
if (yearType.includes("deficient")) {
monthLengthsArr[1] = 29; monthLengthsArr[2] = 29;
} else if (yearType.includes("complete")) {
monthLengthsArr[1] = 30; monthLengthsArr[2] = 30;
} else { // Regular common
monthLengthsArr[1] = 29; monthLengthsArr[2] = 30;
}
}
// Check if the day is valid for the given month
if (hebrewDay monthLengthsArr[monthIndex]) {
return -1; // Invalid day for this month
}
var days = 0;
for (var i = 0; i < monthIndex; i++) {
days += monthLengthsArr[i];
}
days += hebrewDay – 1; // Adjust for 0-indexed days into year
return yearStartJDN + days;
}
// Converts a JDN to a Gregorian date
function jdnToGregorian(jdn) {
var L = jdn + 68569;
var N = Math.floor(4 * L / 146097);
L = L – Math.floor((146097 * N + 3) / 4);
var I = Math.floor(4000 * (L + 1) / 1461001);
L = L – Math.floor(1461 * I / 4) + 31;
var J = Math.floor(80 * L / 2447);
var day = L – Math.floor(2447 * J / 80);
L = Math.floor(J / 11);
var month = J + 2 – 12 * L;
var year = 100 * (N – 49) + I + L;
return {
year: year,
month: month,
day: day
};
}
// Gregorian to JDN
function gregorianToJDN(year, month, day) {
var a = Math.floor((14 – month) / 12);
var y = year + 4800 – a;
var m = month + 12 * a – 3;
return day + Math.floor((153 * m + 2) / 5) + 365 * y + Math.floor(y / 4) – Math.floor(y / 100) + Math.floor(y / 400) – 32045;
}
// — Main Calculator Logic —
function calculateJewishBirthday() {
var birthDay = parseInt(document.getElementById("birthDay").value);
var birthMonth = parseInt(document.getElementById("birthMonth").value);
var birthYear = parseInt(document.getElementById("birthYear").value);
if (isNaN(birthDay) || isNaN(birthMonth) || isNaN(birthYear) ||
birthDay 31 || birthMonth 12 || birthYear < 1) {
document.getElementById("result").innerHTML = "Please enter a valid Gregorian birth date (Day, Month, Year).";
return;
}
// 1. Convert Gregorian birth date to JDN
var birthJDN = gregorianToJDN(birthYear, birthMonth, birthDay);
// 2. Convert JDN to Hebrew birth date
var hebrewBirthDate = jdnToHebrew(birthJDN);
// 3. Find the next Jewish birthday in the Gregorian calendar
var today = new Date();
var todayJDN = gregorianToJDN(today.getFullYear(), today.getMonth() + 1, today.getDate());
var currentHebrewYearToday = jdnToHebrew(todayJDN).year;
var targetHebrewYearForBirthday = currentHebrewYearToday;
var nextBirthdayJDN = -1;
var effectiveHebrewMonthName = "";
// Loop to find the next valid Jewish birthday JDN (check current Hebrew year and next Hebrew year)
for (var i = 0; i = todayJDN) {
break; // Found a valid future birthday
}
targetHebrewYearForBirthday++; // Try next Hebrew year
}
if (nextBirthdayJDN === -1) {
document.getElementById("result").innerHTML = "Could not determine the next Jewish birthday. Please check your birth date.";
return;
}
var nextJewishBirthdayGregorian = jdnToGregorian(nextBirthdayJDN);
// Calculate Hebrew Age
var hebrewAge = currentHebrewYearToday – hebrewBirthDate.year;
// Adjust age if birthday hasn't passed yet in the current Hebrew year
if (nextBirthdayJDN > todayJDN) {
hebrewAge–;
}
// Ensure age is not negative for very recent births
if (hebrewAge < 0) hebrewAge = 0;
var resultHTML = "