Age Calculator

“How old am I?” has an obvious answer and several useful ones. The Age Calculator gives the calendar breakdown in years, months and days, the running totals in days, hours and minutes, the weekday you were born on, and how long until the next birthday. Set the second date to something other than today and it becomes a general interval calculator.

Calendar arithmetic is done by borrowing rather than by dividing. When the day of the end date falls before the day of birth, the calculation borrows the real length of the preceding month, so an interval crossing February is not quietly assumed to be 30 days. That is why the years and months figure agrees with how people actually count birthdays.

Dates are parsed into their year, month and day components directly rather than through a UTC string. The naive route puts the date at UTC midnight and then reads it back in your local zone, which rolls the calendar back a day for everyone west of Greenwich and can report the wrong day of the week. Everything computes on the page.

Getting a Full Breakdown From Two Dates

1. Press Load Sample to fill in a date of birth of 1990-07-04 against today, or set Date of Birth yourself.
2. Leave the second field on today for an age, or change it for any other interval. The date-of-birth picker caps itself at the end date, so the two cannot cross by accident.
3. Read the calendar breakdown first: years, months and days are what a person means by age. The totals below are the same span expressed differently, not extra information.
4. Check Born on a for the weekday and the next-birthday line for the countdown, which is measured from the end date rather than from today whenever you have changed it.
5. Press Copy for a plain-text summary of every field at once, ruled into sections, and watch for the badge confirming the write.

Edge Cases: Leap Days and Impossible Dates

• A 29 February birthday has no anniversary in a common year. The next-birthday date is built from the month and day directly, so it rolls into 1 March, which is the convention most jurisdictions use for legal purposes.
• The hours and minutes figures are derived from clock components rather than the elapsed remainder, so they describe the time-of-day offset between the two dates and not a leftover fragment of a day.
• A date of birth after the end date returns Date of birth must be before the end date. and clears the result rather than showing a negative age.
• Anything more than 150 years before the end date returns Please enter a realistic date of birth (within 150 years of the end date)., which catches a mistyped century far more often than it blocks a legitimate entry.
• Total days counts real elapsed days including every leap day, so it will not equal years multiplied by 365. Over 30 years the gap is 7 or 8 days.
• Daylight saving shifts are absorbed by the local date parsing, so an interval spanning a clock change does not gain or lose an hour in the day count.