Birthday Calculator
This birthday calculator takes your date of birth and turns it into three things people always want to know: the day of the week you were born, your age right now, and exactly how many days until your next birthday. Enter your date of birth and the countdown appears instantly, counted in whole days from today in New Zealand to the date your next birthday falls on, along with the weekday that birthday lands on so you know whether you are celebrating on a weekend or squeezing it into a work day. The day-you-were-born result answers a question most people cannot answer about themselves without looking it up, and it is a fun one to check for children, parents and grandparents too. People use the countdown to plan parties and time off, to build anticipation with kids, to schedule cards and presents that need postage time, or just to settle the question of whose birthday comes around first. If you were born on 29 February, the calculator treats your birthday as 1 March in non-leap years, the day you complete another full year, so the countdown keeps working every year. Everything updates the moment you change the date, and no information you enter leaves your browser.
The countdown uses today's date in New Zealand. Enter a date of birth in the past.
Whole days from today to your next birthday: the day before shows 1 day, and on your birthday it shows 0 days. Age is in completed years. A 29 February birthday counts as 1 March in non-leap years.
How it works
The calculator reads the day, month and year from your date of birth. The weekday you were born comes straight from the calendar, because every date maps to exactly one day of the week. Your age is the number of completed years since your birth: the current year minus your birth year, less one if this year's birthday has not happened yet. For the countdown, the calculator finds the next occurrence of your birthday: this year's date if it is still ahead, otherwise the same date next year, then counts the whole days between today and that date. Today itself is not counted but the birthday is, so the result falls by one each midnight until it reaches 0 days on the birthday itself. For 29 February birthdays the next occurrence is taken as 1 March whenever the year is not a leap year, which keeps both the age and the countdown consistent from year to year.
Worked example
Take someone born on 15 January 1990. The calendar puts that date on a Monday, so that is the day they were born. Their age is the completed years since 1990: once the 15 January birthday has passed in a given year, they have turned the current year minus 1990. The countdown works the same way for any date: if your next birthday falls on 15 January and today is 15 December, there are 31 whole days between the two dates, so the calculator shows 31 days. On 14 January it shows 1 day, and on 15 January it shows 0 days along with the weekday the birthday lands on, which shifts forward one weekday most years and two after a leap day.
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