Dates & birthdays
Find an age on a chosen date, count elapsed days, and see when the next birthday falls.
Age on the as-of date
Enter a date of birth and an as-of date, then select Calculate.
February 29 birth: this calculator uses February 28 for birthday anniversaries in nonleap years, and February 29 in leap years.
Exact calendar-date arithmetic. No birth time is needed. Your dates are calculated in your browser; successful results appear in this tab's Recent calculations.
For someone born on May 15, 1990, enter 1990-05-15 in Date of birth and 2026-09-27 in As-of date. Select Calculate, or use Try an example for these same inputs.
36 years, 4 months, 12 days
This is their age on the reunion date. 436 completed months lead directly from the birth date to September 15, 2026; another 12 days reach September 27. To find their age today instead, select Use today, then Calculate again.
Keep the same birth date and September 27, 2026 as-of date. Read Birthday on or after the as-of date and Birthday countdown and age in the result.
2027-05-15 · in 230 days · 37 years old
The 230 days run from the chosen September 27 date to May 15, not from today on your device. The age above is 36 years, 4 months, 12 days; 37 is the age reached on that birthday. If the as-of date is the birthday itself, the countdown is On the as-of date (0 days).
0 years, 1 month, 0 days
February has no day 31, so its last day completes one calendar month under this tool's rule. Exactly 28 days have elapsed. March 31 completes two months, because each month offset starts from the original January 31.
1 year, 0 months, 0 days
The anniversary is clamped to February 28 in 2025. On March 28, 2025 the result is 1 year, 0 months, 28 days: the 13-month anniversary is March 29, not March 28.
This calculator finds the largest whole number of completed months from the birth date that does not pass the as-of date. Each candidate month is measured directly from the original birth date. The original day number is kept when possible; otherwise that candidate uses the last day of its month. Completed months are divided into whole years and a remainder of 0–11 months, then the remaining calendar days are counted from that anchor to the as-of date.
This is a birth-anchored month convention, not a series of one-month additions. January 31 plus two months is March 31, even if the one-month anniversary was shortened to February 28. Nor are years converted to 365 days or months to 30 days. The residual days can be as high as 30 because the next calendar anniversary may be more than 30 days away.
The next birthday is the first birthday anniversary on or after the as-of date. A birthday on that date has a countdown of zero days and displays “On the as-of date (0 days).” February 29 birthdays use February 28 in nonleap years. If the next birthday would be after year 9999, it is unavailable while the elapsed age remains valid.
All dates use the proleptic Gregorian calendar for years 0001–9999, including dates before its historical adoption. A leap year is divisible by 4, except a century year must also be divisible by 400: 2000 was a leap year; 1900 and 2100 were not. Reference: U.S. Naval Observatory — Leap Years.
Total elapsed days are the difference between the two calendar dates, with no inclusive extra day. The birth date itself is age zero. Days are not measured with timestamps, so daylight-saving changes and time zones do not alter a calculation for the same two dates. Only the initial “today” value uses your device's local date; the tool does not continuously advance it while the page stays open.
Results are exact for the stated calendar convention and require no decimal rounding. Birthday observance and age rules for a legal, school, benefit, or eligibility decision may differ; this tool does not apply those rules.
Months are unequal, and tools can handle month ends or leap-day anniversaries differently. Some apply years first and then months from an adjusted date. This tool measures each completed month from the original birth date and states its February 29 convention. Total elapsed calendar days can agree even when the years/months/days breakdown differs.
Only a birthday anniversary in a nonleap year uses February 28. In a leap year, the birthday remains February 29. Monthly anniversaries also keep day 29 whenever that month contains it.
Yes. Enter the future date in As-of date. It must fall on or after the date of birth, and both dates must be within years 0001–9999.
No. The inputs are calendar dates. Converting elapsed days into hours would not establish an exact birth-time interval, especially across clock changes. This calculator therefore reports date-based age and elapsed days only.
Reset clears the birth date and result, then sets the as-of date to the current local date on your device. It does not add a recent-calculation entry or erase previous entries.