Age Calculator
Work out exact age in years, months, and days between a date of birth and any reference date. Also shows total days lived, day of the week born, and the next birthday.
Age
35 years, 11 months, 1 day
- Total days lived
- 13,119
- Total weeks lived
- 1,874
- Total months lived
- 431
- Day of the week born
- Friday
- Next birthday
- 15 June 2026 (Monday) — 30 days away
Years complete since birth, then additional whole months since the last birthday, then remaining days. Leap-day birthdays roll to 28 February in non-leap years, matching the standard date-library convention.
How to use this calculator
Enter the date of birth (year, month, day) and the reference date you want the age calculated for. Leave the reference date as today for current age, or set it to a future date to find out how old someone will be on that day. The calculator returns age as years, months, and days, plus total days lived, day of the week born, and when the next birthday falls.
How the calculation works
Age is calculated as the number of complete birthday anniversaries between the two dates, then the number of additional whole months since that anniversary, then the remaining days. For example, someone born on 15 June 1990 viewed on 16 May 2026 is 35 years (last birthday 15 June 2025), 11 months (last month boundary 15 May 2026), and 1 day. The algorithm matches PHP DateInterval, date-fns, dayjs, and Wolfram Alpha. All arithmetic uses the proleptic Gregorian calendar in UTC, so timezone and daylight-saving offsets never shift the result.
Worked example
Date of birth: 15 June 1990. As of: 16 May 2026. The most recent birthday anniversary on or before 16 May 2026 is 15 June 2025 — that gives 35 complete years. From 15 June 2025, eleven whole months reach 15 May 2026. From 15 May 2026 to 16 May 2026 is one day. So the age is 35 years, 11 months, 1 day — 13,119 days, or 1,874 weeks, in total. The next birthday lands on 15 June 2026 (a Monday), 30 days away.
Frequently asked questions
How is age calculated when the birthday is 29 February?
In a non-leap year, the birthday anniversary is treated as 28 February. So someone born on 29 February 2000 is considered to turn 1 year old on 28 February 2001, then 2 on 28 February 2002, and so on, until 29 February 2004 when the actual leap day returns. This matches the convention used by date-fns, dayjs, PHP DateInterval, and Wolfram Alpha, and is the rule recognised by the UK Births and Deaths Registration Act 1953 for Scotland. England and Wales use 1 March in some statutory contexts, but 28 February is the near-universal default in everyday and software use.
Why does the result use proleptic Gregorian calendar dates?
The Gregorian calendar was introduced in 1582 and adopted at different times in different countries — Britain switched in 1752, Russia in 1918. To avoid country-specific edge cases, this calculator projects Gregorian rules back into history (the "proleptic" Gregorian calendar). For ordinary use this is the right choice. If you are working with dates earlier than 1582 and need Julian-calendar results for historical research, adjust the input dates accordingly before entering them.
In what order are years, months, and days counted?
Largest unit first: years, then months, then days. Years are counted as complete birthday anniversaries, months are then counted from the most recent anniversary, and days are the leftover remainder. This order matches the convention used in every major date library and produces the breakdown that humans naturally use — "35 years, 11 months, 1 day" rather than "431 months and 1 day".
Why does the total day count differ from years × 365?
Because of leap years. Over a 35-year span there are typically eight or nine leap days, so a true total-days count is usually a little higher than 35 × 365 = 12,775. The calculator counts actual calendar days, including every leap day, so the total is accurate to the day. If you divide the total days by 365.25 (the average length of a Gregorian year, including leap years), you should get a number very close to your age in years.
Can I use a future reference date to see how old I will be?
Yes. Set the reference date to any date in the future. The calculator will report the age you will be on that date, assuming you reach it. This is useful for retirement planning, milestone birthdays, or checking the age cutoff for an event or eligibility threshold. The reference date must be on or after the date of birth — if you swap them, the calculator returns an error rather than a negative age.
Does this calculator handle BCE dates or year zero?
The calculator accepts years from 1 to 9999 in the Common Era. Year zero does not exist in the standard calendar (the year before 1 CE is 1 BCE), and BCE dates are not supported. For historical research involving BCE dates or astronomical year numbering (which does include a year zero), use a specialist tool such as a Julian Day Number calculator instead.