Date Calculator
Add or subtract years, months and days from a date. Get the resulting date, its day of the week, ISO format, and the total number of days and weeks between the two dates.
Input
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
The Date Calculator adds or subtracts a duration to any date and tells you exactly where you land. Enter a start date, choose whether to add or subtract, then set how many years, months, and days to shift by. The result shows the new date, its day of the week, the ISO (YYYY-MM-DD) form, and the total number of days and weeks between the two dates.
It is built for the everyday "what date is X from here?" questions: a 90-day contract deadline, a 6-week follow-up, a project that ships in 2 years and 3 months, or how many days ago something happened.
How to use it
- Pick a Start Date using the date picker.
- Choose the Operation — Add moves forward in time, Subtract moves backward.
- Enter any combination of Years, Months, and Days (leave a field at 0 to skip it).
- Read the result table. It updates automatically as you type.
The output includes the resulting date written out in full, the weekday it falls on, the machine-readable ISO date you can paste elsewhere, and the total elapsed time expressed both in days and in whole weeks plus leftover days.
How the math works
Years and months are applied first as a whole-month shift, then the days are added. When the target month is shorter than the start day, the day is clamped to the last valid day of that month. For example, January 31 + 1 month lands on February 28 (or February 29 in a leap year), not on an overflowed March date. Days are then added as exact 24-hour steps, so the day-of-week and total-days figures stay accurate across month and year boundaries, including leap years.
Why does adding a month sometimes not change the day number?
Calendar months have different lengths. Because the day is clamped to the end of the target month, adding one month to the 31st of a 31-day month can land on the 28th, 29th, or 30th. This matches how people normally reason about "a month later."
Does it handle leap years?
Yes. February 29 is produced only in leap years, and the total-days count uses a true day-by-day calendar, so spans that cross February 29 are counted correctly.
Can I calculate the difference between two dates?
This tool answers "start date ± duration = which date." To measure the exact gap between two known dates (years, months, and days between two birthdays or events), use the companion Age Calculator, which is designed for that direction.
What date format does it output?
Both a human-readable form (for example, Friday, September 25, 1992) and the ISO 8601 form (1992-09-25) that spreadsheets, databases, and APIs expect.
Privacy
Every calculation runs entirely in your browser. Your dates are never uploaded, logged, or stored — there is no server round-trip for the math, so the values you enter never leave your device.