Birthday calculator
| Month | Birthstone | Colour |
|---|---|---|
| January | Garnet | Deep red |
| February | Amethyst | Purple |
| March | Aquamarine | Pale blue |
| April | Diamond | Clear |
| May | Emerald | Green |
| June | Pearl | White |
| July | Ruby | Red |
| August | Peridot | Yellow-green |
| September | Sapphire | Blue |
| October | Opal | Iridescent |
| November | Topaz | Golden |
| December | Turquoise | Blue-green |
Your next birthday is this year if it has not passed and next year if it has, and the panel gives the weekday, the days remaining and the age you will turn. A birthday advances one weekday a year and two across a leap year, so the same date falls on a weekend roughly two years in seven.
How to find your next birthday
Birthdays advance by one weekday each year and by two across a leap year, so the same date cycles through the week on a 28-year pattern and returns to its original weekday four times within it, at gaps of 5, 6, 6 and 11 years. That is why a birthday falls on a weekend roughly two years in seven but unevenly, and why the "it is on a Saturday again" feeling arrives at odd intervals. The whole calendar repeats exactly every 400 years, which is 146,097 days and 20,871 whole weeks, and inside one of those cycles the 13th falls on a Friday 688 times against 684 for a Thursday or a Saturday. Friday really is the commonest day for a 13th, by a margin of four in 4,800.
Working a weekday out in your head is easier than it looks, because a fixed set of dates always shares one. In any year, 4/4, 6/6, 8/8, 10/10 and 12/12 fall on the same weekday, and so do 9/5, 5/9, 11/7, 7/11 and the last day of February. Find that weekday once and every other date in the year is a short count from the nearest of those anchors. Dates before 1752 are answered here proleptically, as though the Gregorian calendar had always been in force; Britain did not adopt it until that year and lost eleven days doing so, so a genuine historical record from before the switch will not agree.
The countdown reads zero on the day itself instead of rolling forward a year, which matches how people talk; nobody says they have 365 days to wait on their birthday. Its largest possible value is 365, on the day after a birthday when a 29 February falls in the year ahead. That date is the awkward one throughout, and no calendar rule decides what to do about it: it is a legal and social convention, and jurisdictions chose differently. The Family Law Reform Act 1969 fixes the English moment at the start of the relevant anniversary, and where the anniversary does not exist, England and Wales take 1 March in practice; some other systems take 28 February. The tabs here use both conventions and say which: the next-birthday tab clamps to 28 February in a common year, while the days-to-the-next-one and since-the-last-one tabs take 1 March, which is what most software does.
The half birthday six calendar months on is the one part of a leapling’s year that behaves, since 29 February plus six months is 29 August, a date that exists every year. A birth date on the 31st is the one that does not: it clamps to the end of a shorter target month, so 31 August gives 28 or 29 February and 31 March gives 30 September. Half birthdays otherwise exist mainly for children whose birthdays fall in the school holidays, and they are not quite the halfway point: six months from 15 January is 181 days and six months from 15 July is 184, because the first half of the year is the shorter one.
Birthstones are the part of this page that is not arithmetic at all. The modern list was standardised commercially in 1912, the traditional lists it replaced still vary by culture, and several months have alternates: alexandrite for June, citrine for November, tanzanite for December, added largely to widen the price range.
What people use it for
- Planning a birthday party
- Counting down to a birthday
- Counting the days since your last birthday
- Finding the weekday you were born on
- Checking which weekday your next birthday falls on
- Giving a summer-holiday birthday a half-birthday date
- Working out how long a couple have been married
- Checking which wedding anniversary is coming up
- Looking up a birth month stone before buying a gift
Questions
This year if it is still ahead, otherwise next year. The date, the weekday and the days remaining are all shown.
The days-to-go row counts from today. On the birthday itself it reads zero rather than rolling forward a year.
Switch to the day-you-were-born tab and enter your date of birth. 15 June 1990 was a Friday.
A common year is 52 weeks and one day, so the date advances one weekday. A leap year advances it two.
Four times every 28 years, at gaps of 5, 6, 6 and 11 years. Where the gaps sit depends on where your birth year falls in the leap cycle.
Roughly two years in seven, but unevenly, because of the leap-year pattern.
Marginally, yes, and it is one of the few pieces of calendar folklore that survives checking. Across the 400-year Gregorian cycle the 13th falls on a Friday 688 times and on a Thursday or Saturday 684 times each, out of 4,800 months. The effect is real and tiny.
Yes. Within any one year, 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 11/7, 7/11 and the last day of February all fall on the same weekday. Establish that day once and count from whichever anchor is nearest your date.
It answers them proleptically, as if the Gregorian calendar had always applied. Britain switched only in 1752 and lost eleven days in the process, so a weekday taken from a contemporary record before then will not match.
The tabs use both conventions and label which. The next-birthday tab clamps the anniversary to 28 February in a common year; the days-to-the-next-one and since-the-last-one tabs take 1 March, which is what most software does. The legal answer is convention rather than calendar: English law fixes the moment at the start of the relevant anniversary, and where 29 February does not exist England and Wales take 1 March, while some other systems take 28 February.
The date six calendar months after your birthday. Born on 15 June, it is 15 December.
Not quite. Six months from 15 January is 181 days and six months from 15 July is 184, because the first half of the year is shorter.
It clamps to the end of the shorter target month: 31 August gives 28 or 29 February, and 31 March gives 30 September. That is the only case where a half birthday moves between years.
The since-the-last-one tab gives the elapsed count, and the two counts always add to a full year: 365, or 366 where a 29 February falls in the stretch ahead. The birthday itself is the only day worth spelling out, because the two are not symmetrical there. The countdown reads zero, since today is the birthday; the elapsed count reads 365, since the last one it can point at is a year ago.
It is set by birth month. April is diamond, July ruby, September sapphire.
The modern list was standardised commercially in 1912. Traditional lists predate it and vary by culture.
Yes: alexandrite for June, citrine for November, tanzanite for December and spinel for August, added largely to widen the price range.
Pearl above all, then turquoise and opal. Pearl is also damaged by perfume and cleaning products.
Yes. Enter their date instead; only the day and month affect the countdown.
Yes. Any annual date works: a wedding, a start date, a warranty. Give it the wedding date and the panel reads out the duration of the marriage in years, months and days, along with the next anniversary due.
Paper is the first, and one of the few that genuinely is traditional. Silver at twenty-five, gold at fifty and diamond at sixty carry the real cultural weight; most of the years in between were filled in by an American jewellers association in 1937.
No. Everything is calculated in your browser.