How to Calculate Your Age in Days, Weeks and Months, With Leap Years

Someone born Jan. 1, 1960 is 24,472 days old on Jan. 1, 2027: 67 years of 365 days plus 17 leap days. The leap day rule comes from the U.S. Naval Observatory (USNO), which says every year divisible by four is a leap year, “except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400.” Below are five worked birthdays, the by-hand method, the Microsoft Excel and Google Sheets formulas, and a free printable worksheet.

Age in days, weeks and months on Jan. 1, 2027

We counted each birthday to Jan. 1, 2027 with a short script on UTC dates and checked each row by hand. Total days is the number of days from the birth date to Jan. 1, 2027. “Leap days” counts each Feb. 29 after the birth date and before Jan. 1, 2027. The two Feb. 29 rows use Mar. 1 as the anniversary in common years, the convention in Scotland’s 1991 law (see below); a Feb. 28 anniversary changes only the leftover days, which become 4.

Birth date	Age (years, months, days)	Total days	Weeks and days	Complete months	Leap days
Jan. 1, 1960	67 y, 0 m, 0 d	24,472	3,496 weeks, 0 days	804	17
June 15, 1980	46 y, 6 m, 17 d	17,001	2,428 weeks, 5 days	558	11
Mar. 1, 2000	26 y, 10 m, 0 d	9,802	1,400 weeks, 2 days	322	6
Feb. 29, 2000	26 y, 10 m, 0 d	9,803	1,400 weeks, 3 days	322	6
Feb. 29, 1960	66 y, 10 m, 0 d	24,413	3,487 weeks, 4 days	802	16

Born Mar. 1, 2000 and Feb. 29, 2000, two people one day apart, differ by exactly one day lived: 9,802 against 9,803. Jan. 1, 1960 to Jan. 1, 2027 is also an exact multiple of seven days, 3,496 weeks.

How do you work out age in years, months and days by hand?

Use the June 15, 1980 birthday and Jan. 1, 2027.

1. Whole years. June 15, 2026 was the last birthday: 2026 minus 1980 is 46 years.
2. Whole months. From June 15, 2026, six months reach Dec. 15, 2026. The seventh month would pass Jan. 1, 2027, so the answer is 6.
3. Days. Dec. 15 to Jan. 1 is 16 days to Dec. 31, plus 1 more: 17 days.

The answer is 46 years, 6 months and 17 days. Always finish with the birthday that has already passed this year. Taking this year minus the birth year without checking the date overstates age for anyone whose birthday has not come yet.

How do you count total days with leap years?

Total days has three parts: 365 for each whole year, one extra day for every Feb. 29 in the span, and the days since the last birthday.

For June 15, 1980 to Jan. 1, 2027:

• 46 whole years x 365 = 16,790 days.
• Feb. 29 fell in 1984, 1988, 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024: 11 leap days. Every one of these years divides by 4. By the USNO rule 2000 counts, because it divides by 400.
• June 15, 2026 to Jan. 1, 2027 is 200 days.
• 16,790 + 11 + 200 = 17,001 days.

Weeks come next: 17,001 divided by 7 is 2,428 with 5 left over. Complete months are 46 x 12 + 6 = 558. For the Jan. 1, 1960 row the same steps give 67 x 365 = 24,455, plus 17 leap days (1960 through 2024), which makes 24,472. Our leap year guide has the rule and every leap year to 2100.

What formulas calculate age in Excel and Google Sheets?

Put the birth date in cell A2. The formulas below are ours, built from Microsoft’s date difference function (DATEDIF) and plain date subtraction. They also use the today’s date function (TODAY), which returns the current date, and the integer function (INT), which rounds down. Microsoft’s own age formulas further down use the year function (YEAR), the current date and time function (NOW) and the month function (MONTH).

What you want	Formula	Check
Complete years	=DATEDIF(A2,TODAY(),"Y")	The Jan. 1, 1960 birthday gives 67 on Jan. 1, 2027
Months after the last birthday	=DATEDIF(A2,TODAY(),"YM")	Microsoft: “The days and years of the dates are ignored”
Total days	=TODAY()-A2	Subtracting one date from another gives days
Whole weeks	=INT((TODAY()-A2)/7)	2,428 for June 15, 1980 on Jan. 1, 2027

Microsoft’s DATEDIF page says the function “is useful in formulas where you need to calculate an age”, and also says it “may calculate incorrect results under certain scenarios”. It adds: “We don’t recommend using the “MD” argument, as there are known limitations with it.” The “MD” argument “may result in a negative number, a zero, or an inaccurate result.” Google Sheets documents “MD” as “the number of days between start_date and end_date after subtracting whole months” and carries no warning on its page. For the leftover days we suggest the by-hand count above, or compare the sheet against it.

Microsoft’s separate Calculate age page gives years as =(YEAR(NOW())-YEAR(A2)) and months as =(YEAR(NOW())-YEAR(A3))*12+MONTH(NOW())-MONTH(A3). Both ignore the day of the month. On Jan. 1, 2027 the years formula gives 47 for a June 15, 1980 birthday against 46 complete years, and the months formula gives 559 against 558 complete months. Our DATEDIF “Y” counts only complete years. For spans between any two dates, see counting days between dates in Excel and Google Sheets.

When does a Feb. 29 birthday count as a year older?

Rules differ by place. The Age of Legal Capacity (Scotland) Act 1991, section 6(2), says: “Where a person has been born on 29th February in a leap year, the relevant anniversary in any year other than a leap year shall be taken to be 1st March.” That applies in Scotland only. We found no written rule in U.S. federal law or for England and Wales.

The U.S. Social Security Administration (SSA) uses a general rule in its Program Operations Manual System (POMS), section GN 00302.400, dated 03/04/2026: “An individual attains a particular age on the first moment of the day preceding the anniversary of his/her birth.” The section does not mention Feb. 29. Check the rule that governs your form, benefit or contract before relying on either date. Total days lived does not depend on any convention, which is why the table’s totals are the safest number to quote.

Print the age in days worksheet

The free one-page worksheet has lines for the two dates, the method as a checklist, a table for the Feb. 29 years in your span, and a table for the answer. It prints the USNO leap year rule at the foot. For date-by-year layouts, our printable yearly calendar and January 2027 calendar help count to a birthday.