What the gap means
The difference between two birth dates is a fixed span — it never changes. What changes is how large it looks relative to the ages involved: four years is most of a lifetime at ages 2 and 6, and unremarkable at 42 and 46. The calculator gives the gap in years, months and days, and in total days.
The two ages that people actually want
Alongside the gap, the result shows how old the older person was on the day the younger was born, and how old each will be on the other's next birthday. Those are usually the numbers behind the question.
Worked example
Between 12 March 1991 and 27 October 1994 the gap is 3 years, 7 months and 15 days — 1,325 days. The first person was 3 years, 7 months and 15 days old when the second was born. Between 12 March and 27 October of any year the difference reads as 3 years; between 27 October and 12 March it reads as 4. Both are the same gap; the whole-year figure just changes twice a year, which is why an exact answer is more useful than a rounded one.
Sibling spacing and school years
A gap of under twelve months usually puts two children in different school years; a gap of just over twelve months might not, because school-year cut-off dates are set by date of birth rather than by the gap. The cut-off varies by state and district. The calculator gives the arithmetic; the cut-off is a local rule you will need to look up.
The convention, stated plainly
The gap is computed the same way an age is: completed years, then completed months, then the remaining days, with the day borrow taken from the actual length of the preceding month rather than a nominal 30. The earlier date is treated as the start, so the answer is always positive and the order you enter the two dates in does not change it. The exact total in days is given alongside, because the calendar figure and the day figure answer different questions and the calendar one is not divisible by anything useful.
Second worked example
Between 20 July 1985 and 14 February 1989 the gap is 3 years, 6 months and 25 days, or 1,305 days. 20 July 1985 was a Saturday; 14 February 1989 was a Tuesday. Note what happens to the whole-year figure across the year: from 20 July to 14 February the difference reads as 3 years, and from 14 February to 20 July it reads as 4. Nothing has changed except which birthdays have passed. This is the single most common source of confusion about age gaps, and it is why a gap is better quoted in days or in full years-months-days than in whole years alone.
Where the answer is genuinely not what people expect
- The gap never changes. It is fixed at the moment the second person is born. What changes is the ratio: four years is enormous at ages 2 and 6 and unremarkable at 42 and 46, which is why the same gap feels different at different times and why the ratio, not the gap, is the thing people are actually reacting to.
- Whole-year figures flip twice a year. Between the two birthdays the whole-year gap reads one higher than it does the rest of the year. Both readings are correct.
- A leap-day birth date. Where one of the two dates is 29 February, the months-and-days part of the gap depends on the leap-year rule applied to the other date's year. We clamp to 28 February in a common year and say so.
- Different calendars. A birth date recorded under a non-Gregorian calendar has to be converted before it can be compared. This tool assumes both dates are already Gregorian.
Twins, terms and the two-week problem
A gap of a few days or weeks is the case where the calendar answer is least useful and the day count is most useful. Two children born eleven days apart have a gap that will never appear in a years-and-months figure at all, but eleven days can put them on opposite sides of a school-year cut-off or a benefit threshold. The result panel always shows the total in days for this reason, and it shows each person's age on the other's next birthday, which is usually the number behind the question when the gap is small.
Related calculations
For one person's age rather than a gap, use the age calculator, which uses the same borrow rule and also reports total days, weekday of birth and days to the next birthday. For a plain span between two dates with both the inclusive and exclusive counts, use days between dates. To project a date a number of years forward, use years from today or the date calculator, both of which clamp 29 February to 28 February in a common year in the same way.