Date Difference Calculator
Find the difference between two dates, add or subtract days, or calculate your exact age.
All date calculations use proper calendar logic — leap years, month-end rollovers, and timezone-safe local dates are handled correctly.
Base date to add or subtract from
Positive = add, negative = subtract
About This Date Difference Calculator
This free online date calculator lets you find the exact number of years, months, and days between any two dates, add or subtract any number of days from a starting date, and calculate a precise age from a birthdate — including the next birthday countdown.
Unlike simple millisecond subtraction, all year/month breakdowns use proper calendar logic that correctly handles leap years (February 29), month-end roll-overs (e.g. January 31 + 1 month = February 28), and other edge cases. No data ever leaves your browser.
How is the year/month/day breakdown calculated?
The tool increments years and months on the calendar directly, not by dividing total milliseconds. For example, from 2024-01-01 to 2025-01-01 it correctly reports 1 year and 366 total days (because 2024 is a leap year), without rounding errors.
What happens when the end date is before the start date?
The Difference mode shows the absolute result and adds a note that the end date precedes the start. The total days figure is shown as a negative number so you can see the direction at a glance. Use the ⇄ Swap button to reverse the direction.
Does it account for leap years?
Yes. February 29 is recognised on leap years. When you add months and the resulting day doesn't exist (e.g. Jan 31 + 1 month → Feb 31), the date rolls back to the last valid day of the month (Feb 28 or Feb 29 on a leap year).
Can I include a time component?
Yes — the Difference mode has optional time fields. When you fill both time fields, the result also shows total hours. If you leave them empty, the calculation is date-only.
How does the next birthday countdown work?
The Age mode finds the next occurrence of your birth month and day in the current or upcoming year. If today is your birthday it shows 0 days remaining. The countdown is shown in total days and also broken into months and days.
What does "Add / Subtract" mode do?
Enter a start date and a number of days. Positive numbers move forward in time; negative numbers move backward. The result shows the destination date and its weekday name. This is useful for deadline planning, e.g. "What date is 90 days from today?"
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