Date Calculator. Calendar math, done right.
Defaults to today, change it to find the age on any date.
Calculated locally, nothing is sent anywhere.
Calendar math, done right
Months and years have different lengths, and leap years shift February, so “3 months from now” isn’t simply 90 days. This calculator uses true calendar arithmetic for adding, subtracting and measuring ages (clamping to the end of shorter months instead of rolling over), and can count working days for project planning. It all runs in your browser: fast, private and free forever.
How to use the Date Calculator
This calculator does three kinds of calendar math: the gap between two dates, a date moved forward or back by a set amount, and an exact age with a countdown to the next birthday. It works with real month lengths and leap years rather than fixed 30 day months. Use it for deadlines, notice periods, project plans and ages.
Step by step
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Pick a tab
Choose Difference to measure the gap between two dates, Add / Subtract to move a date, or Age for an exact age. Each tab keeps its own fields, and the result updates as you type with no button to press.
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Measure a gap
On Difference, set From date and To date. From date starts at today. If the dates are in reverse order, the tool still measures the gap and says so in the sentence under the result.
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Count working days
Check the box Count business days only (Mon-Fri) to count weekdays instead. The count leaves out the start date and includes the end date, so Monday to the next Monday is 5 working days. Public holidays are not removed.
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Move a date
On Add / Subtract, set the Start date (today by default), choose Add (+) or Subtract, and fill in any of Years, Months, Weeks and Days. Years and months are applied first, then weeks and days.
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Find an age
On Age, enter the Date of birth. Leave Age on date empty to use today, or set it to find the age on any other day, past or future.
Reading the result
- Difference shows the gap in years, months and days, then Total days, In weeks (for example 3 w 2 d) and Total hours, which is the day count times 24.
- With business days checked, the headline is the number of working days, with Calendar days and Weekend / non-working days beside it.
- Add / Subtract shows the Result date with its Weekday, the Net days from start and the ISO date in year, month, day order. When the target month is shorter, the day is set to its last day, so January 31 plus 1 month is February 28, or 29 in a leap year.
- Age shows the exact age in years, months and days, plus Total months, Total days and the days until the Next birthday. In years that are not leap years, the countdown moves a February 29 birthday to March 1. On the birthday itself the tile reads Today.
Tips and common mistakes
- Use whole numbers in Add / Subtract. Anything after the decimal point is dropped, so 1.5 months counts as 1 month.
- Adding 3 months is not the same as adding 90 days. Use Months for contract and notice periods so the calendar decides the length.
- Negative numbers work in Add / Subtract too. Adding -2 weeks gives the same date as subtracting 2 weeks.
- Total hours ignores daylight saving changes, because it is calendar days times 24.
- If the Age tab says the date of birth is after the comparison date, check the year in both fields.
Privacy and limits
All date math runs in your browser, using your device's calendar and time zone. Nothing is sent anywhere and nothing is saved, so From date and Start date go back to today when you reload.
Every date question, answered instantly.
Frequently asked questions.
Is the Date Calculator free?
Is it private? Are my dates uploaded anywhere?
How are months and years counted when adding time?
What counts as a business day?
How are February 29 birthdays handled?
Published · Last updated .
