How much older is one person than another? The question sounds like simple subtraction, but dates resist it, because years contain leap days and months contain 28, 29, 30, or 31 days. Subtracting year from year, month from month, and day from day works only when every later component is larger than the earlier one; otherwise, borrowing is needed, just as in ordinary subtraction, except that the amount borrowed depends on which month you borrow from. The Age Difference Calculator handles that borrowing, but understanding the method lets you check its result and spot an error in a date you typed.
The method works from the largest unit down. First, subtract the years. Then subtract the months, borrowing a year, which is 12 months, if the month subtraction goes negative. Then subtract the days, borrowing a month, whose length is the number of days in the month preceding the later date, if the day subtraction goes negative. That last rule is the one people most often get wrong: the borrowed month is not always 30 days, and it is not the month of the earlier date. It is the month you borrow from, which is the month before the later date in the calculation. Working an example slowly makes the rule concrete, and it is worth doing once by hand before trusting any calculator with an important date.
Age difference and age itself are related but distinct. A person's age on a given date is the difference between their birth date and that date, calculated by the same method. The difference between two people's ages is the difference between their birth dates, and it stays constant throughout their lives, apart from the obvious point that on a birthday, one person's age increments while the other's does not yet, so the difference in completed years can appear to change by one for a short period. Stating the difference in years, months, and days avoids that confusion, because it describes the gap between the birth dates themselves, not the two ages on a particular day.
A worked example
Suppose one person was born on 15 March 1990 and another on 20 September 1995. Calculate the difference from the earlier date to the later date. Years first: 1995 minus 1990 equals 5 years, taking us from 15 March 1990 to 15 March 1995. Months next: September minus March equals 6 months, taking us from 15 March 1995 to 15 September 1995, with no borrowing needed because September is later in the year than March. Days next: 20 minus 15 equals 5 days, taking us from 15 September 1995 to 20 September 1995. The total difference is therefore 5 years, 6 months, and 5 days. A check by addition confirms it: start at 15 March 1990, add 5 years to reach 15 March 1995, add 6 months to reach 15 September 1995, and add 5 days to reach 20 September 1995, which is the later birth date exactly.
A borrowing example shows the harder case. Suppose the dates are 20 September 1995 and 15 March 1990, calculated in the other direction, or equivalently, suppose you need the difference between 15 March 1990 and 10 January 1995. Years give 1995 minus 1990, which is 5, but months give January minus March, which is negative 2, so borrow 12 months: years become 4, and months become negative 2 plus 12, which is 10, taking the running date to 15 January 1995 after 4 years and 10 months from 15 March 1990. Days then give 10 minus 15, which is negative 5, so borrow one month: months become 9, and days become negative 5 plus the days in the month before January 1995, which is December 1994 with 31 days, giving 26 days. The result is 4 years, 9 months, and 26 days, and adding it back, 15 March 1990 plus 4 years is 15 March 1994, plus 9 months is 15 December 1994, plus 26 days is 10 January 1995, confirming the calculation. Note that the borrowed month had 31 days because December has 31 days; borrowing from February would have given 28 or 29 days instead, which is why the month matters.
Common mistakes
The first mistake is subtracting days as if every month had 30 days, which gives a plausible but wrong day count in most cases. The second mistake is forgetting leap years when counting total days: a span that includes 29 February has one extra day, and a total-days figure that ignores it will be one day short for each leap day crossed. The third mistake is entering dates in the wrong order, day and month swapped, especially where formats differ between countries; always confirm whether a date is written day-first or month-first before calculating. The fourth mistake is confusing age difference with age on a particular date, and reporting a difference that changes on birthdays. The fifth mistake is using a calculator result for a legal purpose, such as eligibility, without checking the official rule, because legal age can depend on jurisdiction, on time of day, or on how a birthday is counted, and a general calculator does not embody those rules.
Total days are a useful cross-check, but they do not replace years, months, and days. A difference of 5 years, 6 months, and 5 days spans a different number of total days depending on which years and months it covers, because of leap days and month lengths. If a calculator gives both forms, check that they are consistent for the specific dates involved, not merely plausible in general. For anniversaries and deadlines, also check whether the counting is inclusive or exclusive of the start date, since that convention shifts a result by one day and causes more arguments than any other date error.
A practical routine
A routine that works for most date differences has five steps. First, write both dates in full, with month names, to avoid format confusion. Second, subtract years, then months, then days, borrowing as shown above. Third, add the result back to the earlier date to confirm it reaches the later date exactly. Fourth, if a total-days figure is needed, count it separately, including leap days. Fifth, for any legal or official use, check the applicable rule rather than relying on the calculator alone. For related date work, the Age Calculator finds a person's age on a given date, and the Date Calculator adds or subtracts days from a date. For another date walkthrough, the guide iCloud Photo Date Guide explains how dates are stored and displayed in photo libraries.
An age difference is years, then months, then days, with borrowing from the correct month. Work it slowly once, check it by adding back, and the calculator's answer will be a confirmation, not a mystery.