Age Calculator — Full Years, Leftover Months and Days, and the Next Birthday
Type a birthday and read the age the way people actually say it: full years plus the leftover months and days, how many days that adds up to, how long until the next birthday, which weekday the birth date fell on, and the zodiac sign of that year.
Pick a birthday. The second box is only for checking the age on some other date — for example on a contract start or an old school photo.
How the age is counted
Age is not one number: years, days, and days to go
An age is not one number. A form wants the full years, a parent wants “he is two and a half”, a lawyer or a doctor wants the count in days, and a birthday reminder wants the other direction — how long until it. All four come from one date, plus the weekday the birth date fell on and the zodiac sign of that year.
The second date box is what makes it usable for other people’s records: fill in a date of birth and a date in the past and it reads the age as it stood then.
How the age is counted
Full years increments on the birthday itself, so a person born in 1990 is 35 full years old from 2025-10-03 onwards and 34 up to and including the day before.
Age spelled out is a calendar breakdown, not a decimal. The month step counts whole months forward from the birthday, and a month with no matching day number lands on its last day; only what is left over is read as days. That is why the line reads “36 years 4 months 18 days” instead of folding months into a nominal 30 days — real months run from 28 to 31 days, so the remainder is stated in days.
Days since birth counts calendar days between the two dates at local midnight. Counting from midnight to midnight keeps daylight saving out of the result: a stretch that crosses a clock change is still exactly as many days as it looks.
Whole months since birth is the same breakdown multiplied out to months, so it stays consistent with the full-years figure.
The next birthday and the round one
The next birthday is the coming anniversary of the birth date. For a 29 February birthday, the anniversary in an ordinary year falls on 28 February — that is the rule most calendars and reminder apps use, and the result line says when it has been applied.
The round-number birthday is the next multiple of ten. Someone who is 34 is counting down to 40, not to 35.
The zodiac sign
The sign is chosen from the Gregorian year of the birth date, which is the shortcut most year tables take. The Chinese zodiac year traditionally changes with the lunar new year, in late January or February, so a birth in the first weeks of the year can belong to the previous sign. Check a lunar calendar for those dates; no lunar calendar data feeds the zodiac step, which reads the Gregorian year only.
Questions
Does the birthday get sent anywhere? No. The date boxes are read by the script running in the browser and every number comes from the browser’s own calendar — what you type never leaves this device.
Why is the age on the form different from this? Forms almost always want full years only. Use the first box and ignore the breakdown; the two are computed from the same rule.
Can it count forward to a birthday instead? Put today’s date in the second box and read the “days until the next birthday” figure. For any other target date, the date adding page works the other way round.