Age & Date Calculator
Find an exact age from a birthday (years, months, days), the duration between two dates, or add and subtract days, weeks, months, and years from any date.
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Disclaimer: This free tool is provided “as is,” without warranties of any kind, and is for general informational purposes only — not professional, legal, financial, medical, tax, or engineering advice. Results may contain errors; verify anything important independently and use at your own risk. We accept no liability for any loss or damage arising from its use. See our Terms of Use for details.
Step-by-Step Guide
Choose one of three modes.
Age mode: enter a birth date. The tool calculates exact age as of today (or a reference date you choose) in years, months, and days. It also shows total days lived, approximate total weeks, months, and hours, the day of the week you were born, and a live countdown to your next birthday.
Days Between mode: enter two dates. The tool shows the total calendar days between them, the number of weekdays (business days, Monday through Friday), and the number of complete weeks.
Add/Subtract mode: enter a start date and specify an amount and unit (days, weeks, months, or years) to add or subtract. The resulting date is shown with its day of the week. Enter a negative amount to go backwards.
Age mode — born 1990-01-15, as of 2025-06-20: age = 35 years, 5 months, 5 days. Total days lived = 12,940. Next birthday in 209 days (January 15, 2026).
Days Between — Jan 1, 2024 to Dec 31, 2024: 365 total days, 261 weekdays, 52 complete weeks (2024 is a leap year).
Add/Subtract — starting March 15, 2025 + 90 days = June 13, 2025 (Saturday).
Who it's for
Anyone checking ages, planners, HR staff, parents, and people counting down to events.
Core Features
- Exact age in years, months, and days, plus total days, weeks, months, and hours lived.
- Day-of-the-week born and a live countdown to the next birthday.
- Days-between-two-dates mode including a business-days (weekday) count.
- Add or subtract days, weeks, months, or years from any starting date.
🛡️ No tracking — your inputs, keys, and details never leave this client sandbox.
How is exact age calculated in years, months, and days?
Exact age is computed by counting full years first (incrementing the birth year until adding another year would exceed today), then counting full months in the remaining period, then counting remaining days. This handles leap years and month length differences correctly — for example, a person born on February 29 in a leap year turns a year older on March 1 in non-leap years.
What counts as a business day in the Days Between mode?
Business days count Monday through Friday only — Saturday and Sunday are excluded. The tool does not account for public holidays, which vary by country, industry, and company. For an accurate business-day count including specific holidays, you would need to specify which holidays to exclude.
Why does adding months sometimes give a different day of month?
When you add months to a date and the resulting month has fewer days than the original, the date is clamped to the last day of the target month. For example, January 31 + 1 month = February 28 (or 29 in a leap year), because February does not have 31 days. This follows standard calendar arithmetic conventions used by most date libraries and spreadsheet programs.
What day of the week was I born?
The tool creates a JavaScript Date object from your birth date and calls getDay(), which returns a value from 0 (Sunday) to 6 (Saturday). The browser's Date implementation uses the proleptic Gregorian calendar internally. Enter your birth date in Age mode and the day of the week appears alongside the other results.
Why is it so hard to calculate an exact age?
An age seems like simple subtraction: current year minus birth year. But try to calculate the exact age of someone born on January 31, 2000, as of March 1, 2025 — down to years, months, and days — and the simple subtraction breaks immediately. Do they turn 25 in January, or have they been 25 for one month and one day? What counts as a "complete month" when February has 28 days and January has 31? The answer requires a formal algorithm, not arithmetic intuition.
How exact age is calculated
The correct algorithm works by counting complete units from largest to smallest. First, count full years: how many times can we add a year to the birth date without exceeding today? That gives the years component. Then, within the remaining partial year, count full months using the same logic — how many times can we add a month without exceeding today? The remaining days are the difference between the resulting date and today. This approach handles all the edge cases: a person born on January 31 cannot have a "complete month" of February because February doesn't have 31 days, so the next complete month after January 31 is March 1.
The leap year birthday problem
People born on February 29 — a date that exists only in leap years — present a specific edge case. The standard convention is that they celebrate their birthday on February 28 in non-leap years (reaching the last day of February) or March 1, depending on jurisdiction. In the US, legal age milestones for people born on February 29 are typically calculated as of March 1 in non-leap years. This tool follows the most common computing convention: the birthday is treated as having occurred at the end of February in non-leap years.
Business days: a useful approximation with real limits
The weekday count between two dates — excluding Saturdays and Sundays — is a useful proxy for working days in project planning and deadline estimation. It is not a substitute for a proper business-day calendar that accounts for public holidays, which vary by country, state, and industry. For rough planning, it works well. For contractual deadlines, legal filings, or financial settlement calculations, an authoritative holiday calendar is essential and the weekday-only count can lead to meaningful errors.