Years, then months, then days — each borrowed from the next
An exact age is a countdown, not a subtraction. You take the full years between the two dates, then the whole months that remain, then the leftover days. When the current day falls earlier in the month than the birth day, the days are borrowed from the previous month, whose length changes the result.
The same span in other units
The years-months-days breakdown is only one view. The same elapsed time can be stated as a running total, which is handy for milestones and record-keeping.
| Unit | How it is counted |
|---|---|
| Total months | years × 12 + months |
| Total weeks | total days ÷ 7, rounded down |
| Total days | exact day count between the two dates |
| Next birthday | the birth date in the following year |
Total days is the ground truth; the weeks and months figures are derived from it, so they always agree.
Why months and leap years make it fiddly
- Months are not equal. January, March, May, July, August, October and December have 31 days; April, June, September and November have 30; February has 28 or 29. The borrowed-day step uses the actual length of the previous month.
- Leap years add a day. A year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400. That single extra day in February is why a plain year-subtraction drifts.
- The precision matters in practice. Legal age of majority, school enrollment cutoffs, insurance age bands and pediatric records can all turn on a difference of a few days.
Common questions
Why can't I just subtract the birth year to get my age?
Subtracting years ignores the months and days, which can be almost a full year off. Months also vary from 28 to 31 days and leap years add a day, so an exact age counts down through years, then months, then days.
How are the years, months and days worked out?
First count the whole years, then the whole months left over, then the remaining days. If the current day is earlier in the month than the birth day, the calculation borrows days from the previous month, exactly like subtracting by hand.
When do people born on February 29 have a birthday?
In common years, leap-day birthdays are usually marked on February 28 or March 1. Age calculators normally treat February 28 as the birthday for counting purposes, which can affect the precise moment someone reaches a given age.
Why does my insurance age differ from my real age?
Many life insurers use "age nearest birthday" rather than "age last birthday." If you are more than six months past your last birthday, they count you as a year older, which can move your rate band.
