Subtracting two dates component by component looks correct until the birth day is larger than the current day. Then you borrow, and the borrow rule decides whether your age in years, months, and days is right. Our age calculator's rule is simple enough to state in four steps, and it carries a real edge case on the 31st that you should know about before you use any result.
The algorithm, step by step
1. Subtract components: years = current year minus birth year, months = current month minus birth month, days = current day minus birth day.
2. If days is negative, subtract one from months and add the length of the month before the current month to days.
3. If months is negative, subtract one from years and add 12 to months.
4. Compute elapsed totals from the millisecond difference: totalDays divides by 86400000, totalHours by 3600000, totalMinutes by 60000, each floored.
Step 2 is the entire subtlety. The length added is the length of the month before the current month, regardless of your birth month. That single choice makes some inputs correct and others negative.
A worked age, end to end
Birth date 15 June 1990, evaluated on 24 September 2026:
1. years = 2026 - 1990 = 36. months counts from zero, so September is 8 and June is 5: months = 8 - 5 = 3. days = 24 - 15 = 9.
2. No borrow fires. Age: 36 years, 3 months, 9 days.
3. Totals: 13250 days, 318000 hours, 19080000 minutes.
4. Next birthday: 15 June 2026 has already passed, so the tool targets 15 June 2027, which is 264 days out.
Notice that the result carries two different clocks. The years, months, and days count calendar anniversaries. The totals count elapsed milliseconds between the two dates, exact to the moment you click. A result like 13250 days with 3 months and 9 days is consistent, because the two clocks measure different things.
The borrow rule and the 31st problem
The borrow adds the length of the month before the current month against whatever your birth day is. It breaks when your birth day exceeds that length.
Verified failure. Birth date 31 January 2000, evaluated on 1 March 2026:
1. months = 2 - 0 = 2. days = 1 - 31 = -30.
2. The borrow fires: months drops to 1, and February 2026 has 28 days, so days = -30 + 28 = -2.
3. Output: 26 years, 1 month, negative 2 days.
The failure conditions are deterministic for this implementation:
1. Birth days 1 through 28 never produce a negative day count.
2. Birth day 29 goes negative only against a 28-day February.
3. Birth day 30 goes negative against February in any year.
4. Birth day 31 goes negative against every month with fewer than 31 days, which is every month the borrow might hand you except July and August.
If you see a negative day count, fix it by hand. Under the usual anniversary convention, the example above is 26 years, 1 month, and 1 day, treating 28 February as the February anniversary of a 31 January birth date. The convention, and whether a jurisdiction accepts it, is outside what the tool computes.
Leap day birthdays
The next-birthday step constructs a date from the birth month and birth day in the current year. When the birth day is 29 February and the year is not a leap year, that constructor rolls over to 1 March. Verified against the component's own construction: 29 February in 2025 yields 1 March 2025.
So our tool counts your non-leap birthday as 1 March. If you celebrate on 28 February instead, read the countdown as one day more than displayed. The years, months, and days breakdown is unaffected, because it never constructs the birthday.
Practical uses that demand the exact figure
1. Eligibility cutoffs for school, sport, and benefits compare a birth date against a fixed deadline. Verify the deadline convention, completed age or reached age, before trusting any tool.
2. Insurance and medical dosing often price by completed months. Compute the month count as years times 12 plus months, and ignore the day remainder when the contract says completed months.
3. Employment milestones, such as vesting and probation, usually follow calendar anniversaries. The leap day rollover above applies to 29 February hires.
Honest downsides
- Negative day counts are reachable, as shown. Sanity-check the sign before using a result for anything official.
- There is no time-of-day input. The calendar breakdown uses dates only, while the elapsed totals include the current time of day, so the two clocks can disagree about whether a boundary has passed.
- Two age calculators can both be correct and still disagree, because completed-months conventions differ. Name the convention you need before you compare results.
- A future birth date produces no output at all. The function returns before rendering anything.
Checklist
- [ ] Enter the birth date and check the day-of-month sign in the result.
- [ ] If the birth day is 29, 30, or 31, verify the borrow month length yourself.
- [ ] For 29 February births, decide whether the non-leap anniversary is 28 February or 1 March.
- [ ] Name the convention the requester needs: completed age, reached age, or elapsed time.
- [ ] Cross-check one total, such as days, against a second source before a legal or medical use.
Run your own birth date through the [age calculator](/en/age-calculator) and check the day component against a hand count. If your date lands on the 29th, 30th, or 31st and the tool disagrees with your arithmetic, send both results, because those boundary inputs are exactly where this algorithm needs the most testing.
