Birthday Calculator

This page is about the birthday itself: the weekday it fell on, when it comes around next and which dates along the way deserve a celebration. For age as of any date, the Age Calculator is the lighter tool. Every example below can be reproduced with the calculator above.

How the calculator finds the day you were born

Every date is turned into a Julian Day Number, a running count of days used in astronomy. Because the count rises by exactly one per day, the weekday is its remainder after dividing by seven: weekday = (JDN + 1) mod 7, where 0 is Sunday. March 14, 1996 has the day number 2,450,157, and 2,450,158 mod 7 = 4, a Thursday. Days lived, countdowns and milestones are then simple subtraction and addition. Exact age follows the same rule as the sister Age Calculator: a month is complete on the same day of the month as the birthday, or on the 1st of the next month when that day does not exist, so a January 31 birthday completes its first month on March 1.

Zeller's congruence: the weekday formula you can do by hand

Before computers, the standard paper method was Zeller's congruence, a 19th-century formula that works for any Gregorian date:

q is the day, m the month, K the last two digits of the year and J the century. January and February count as months 13 and 14 of the previous year. For March 14, 1996: h = (14 + 10 + 96 + 24 + 4 + 95) mod 7 = 243 mod 7 = 5, a Thursday.

Worked example: born on Pi Day, March 14, 1996

Enter March 14, 1996 and count up to October 10, 2026. The calculator returns:

• Born on a Thursday. March 14 is a "doomsday" date, and 1996's doomsday was a Thursday.
• Exact age: 30 years, 6 months, 26 days, which is 11,167 days, 1,595 weeks and 2 days, 366 months or 268,008 hours.
• Next birthday: Sunday, March 14, 2027, in 155 days, turning 31. Next half-birthday: Tuesday, September 14, 2027, in 339 days.
• Pisces, Chinese zodiac Fire Rat (Lunar New Year was February 19, 1996), birthstones aquamarine and bloodstone.

Worked example: January 28, 2006 and the Lunar New Year trap

For a January birthday, Zeller's formula needs m = 13 and the year 2005: q = 28, K = 5, J = 20, so h = (28 + 36 + 5 + 1 + 5 + 100) mod 7 = 175 mod 7 = 0, a Saturday. Using m = 1 and 2006 would give the wrong day.

The same date is a classic Chinese zodiac mistake. Charts list 2006 as the Year of the Dog, but the animal changes on Lunar New Year, January 29, 2006. Born one day earlier, this person is a Rooster (Wood Rooster, Yi-you). On October 10, 2026 they are 20 years, 8 months and 12 days old, exactly 7,560 days or 1,080 weeks, with their next birthday on Thursday, January 28, 2027, 110 days away.

Days until your birthday, half-birthdays and February 29

The countdown does not include the starting day, so a birthday tomorrow is 1 day away and a birthday today reads Today. A half-birthday is six calendar months later; when that month is shorter, the date slides to its last day, so August 29, 30 and 31 birthdays share a half-birthday on February 28 in a common year.

Someone born on February 29, 2004 (a Sunday) has a real birthday only in leap years; the next is Tuesday, February 29, 2028. In common years the calculator rolls the date to March 1, so on October 10, 2026 the countdown shows Monday, March 1, 2027, 142 days away. Exact age follows the same rule: on February 28, 2026 the person was 21 years, 11 months and 30 days old, turning 22 on March 1.

Your birthday's weekday from year to year

A common year is 52 weeks and 1 day, so a birthday moves one weekday later each year, or two after a February 29. Between 1901 and 2099 the pattern repeats every 28 years, as the March 14, 1996 birthday shows:

For mental math, use John Conway's doomsday rule. Each year, 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7, 3/14 and the last day of February share one weekday, as does January 3 (January 4 in leap years). Find the year below and count from the nearest anchor: in 2006 the doomsday is Tuesday, so January 31 is a Tuesday and January 28 a Saturday.

Milestones: 10,000 days, a billion seconds and more

Round numbers of days, weeks and seconds make good excuses for a party. In years they arrive at: 10 million minutes at 19.01, 1,000 weeks at 19.17, 10,000 days at 27.38, 1 billion seconds at 31.69, 20,000 days at 54.76 and 2 billion seconds at 63.38. Week milestones always land on the weekday you were born. For the March 14, 1996 example, assuming a midnight birth:

Zodiac signs, Chinese zodiac and birthstones

The calculator adds three cultural labels without interpretation. Zodiac signs use the common fixed date ranges; tables can differ by a day near a boundary, so the tool flags boundary dates. As NASA notes, astrology is not astronomy and its claims are not based on scientific evidence. Birthstones follow the Gemological Institute of America (GIA).

The Chinese zodiac animal changes on Lunar New Year, between January 21 and February 20, and the element (wood, fire, earth, metal, water) changes every two years. Recent dates from the Hong Kong Observatory tables; a January or February birth before the listed date belongs to the previous animal:

How to use it, and who it is for

1. Enter the date of birth. The three example buttons load the dates used on this page.
2. Leave "count up to" on today, or pick another date to see the age and countdown as of that day.
3. Add a time of birth if you know it; it only affects the hour, minute and second milestones.
4. Scroll down for the details table, milestones, your next 12 birthdays and past birthday weekdays.

Parents use it to plan parties that land on a weekend, friends to spot a 10,000-day or billion-second celebration, genealogists to check an ancestor's birth weekday against a family record, and teachers to show how modular arithmetic turns a calendar into a formula.

Key terms

• Julian Day Number: a continuous day count; the difference of two is an exact number of days.
• Doomsday: the weekday shared by 4/4, 6/6, 8/8, 10/10, 12/12 and the other anchor dates of a year.
• Half-birthday: the date six calendar months after a birthday.
• Lunar New Year: the first day of the Chinese lunar year, when the zodiac animal changes.

What changes the result

Time zones matter near midnight: a baby born at 11:30 p.m. in California was born on the next calendar day in New York. Daylight saving time can shift minute and second milestones by an hour, which the calculator ignores. Very old dates are another case: Britain and its American colonies switched to the Gregorian calendar in 1752, when September 2 was followed by September 14. George Washington was born on February 11, 1731 by the old calendar, which is February 22, 1732 in today's calendar, a Friday. The calculator reads every date as Gregorian.

Tips and limitations

• Double-check the year: 2096 instead of 1996 still returns a valid weekday, just for the wrong century.
• For a January or February birth, read the Lunar New Year note before trusting a printed Chinese zodiac chart.
• Results use whole local calendar days, except the milestones when you enter a time of birth.
• Signs, animals and birthstones are traditions; the tool makes no claims about personality or luck. The Chinese zodiac covers 1901 to 2100.

Related calculators

• For age as of any date, use the Age Calculator; to compare two ages, the Age Difference Calculator.
• To count the days between two dates, use the Date Calculator; for a live countdown, the Countdown Calculator.
• For the Julian Day Number itself, use the Julian Date Converter; for a just-for-fun chart from your birth date, the Biorhythm Calculator.

Sources

Weekdays, ages and day counts are exact calendar arithmetic. The reference data comes from:

• U.S. Naval Observatory, Leap Years (the Gregorian leap-year rule).
• Hong Kong Observatory, Gregorian-Lunar Calendar Conversion Tables (Lunar New Year dates, 1901 to 2100).
• Gemological Institute of America, Birthstones by month.
• NASA Space Place, What Are Constellations? (astronomy versus astrology).
• National Archives, George Washington's Birthday (the 1752 calendar change).
• Fliegel, H. F. and Van Flandern, T. C., "A Machine Algorithm for Processing Calendar Dates", Communications of the ACM 11(10), 1968.