Date Calculator

Add or subtract days, weeks, months, and years from any date

Date Calculator

Calculate the difference between dates

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Date Calculator
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What Is a Date Calculator?

A date calculator lets you perform arithmetic on dates — adding or subtracting a number of days, weeks, months, or years to find a resulting date. It's an essential tool for planning deadlines, calculating due dates, scheduling projects, figuring out what day of the week an event falls on, and any situation where you need to know what comes a specific amount of time before or after a reference point.

This tool handles all common date operations: adding time to a date, subtracting time from a date, and finding the date N working days from now. It automatically accounts for varying month lengths — February's 28 or 29 days, 30-day months versus 31-day months — and leap years, so you always get an accurate result without doing the mental math yourself.

How to Use the Date Calculator

  1. Select a start date — it defaults to today, but you can pick any date in the past or future.
  2. Choose whether you want to add time to that date or subtract time from it.
  3. Enter the number of years, months, weeks, and/or days you want to shift by.
  4. Click Calculate to see the resulting date, including the day of the week it falls on.

Date Arithmetic Formulas

Date addition: Result = Start date + Duration Date subtraction: Result = Start date − Duration Month handling: Adding 1 month to Jan 31 → Feb 28 (or 29 in leap year) — month-end clamping applies Leap year check: Year is leap if: (divisible by 4) AND NOT (divisible by 100) OR (divisible by 400)

Adding months uses calendar month boundaries, not 30-day increments. Adding 1 month to March 31 gives April 30 — the result is clamped to the last day of April since April only has 30 days. This matches how banks, contracts, and legal documents interpret "one month later."

Worked Examples

Project deadline: today + 90 days

If today is April 27, 2026 and your project is due in 90 calendar days, add 90 days to April 27. Result: July 26, 2026 (a Sunday). If you need the deadline on a business day, move it to the nearest Friday — July 24 — or Monday — July 27.

Contract term: April 1 + 6 months

A service contract signed on April 1, 2026 for a 6-month term expires on October 1, 2026. Adding 6 months shifts the month value by 6 (April = month 4, + 6 = month 10 = October) while keeping the day value at 1.

Looking back: what date was 100 days ago?

To find the date 100 days before today (April 27, 2026), subtract 100 days. Counting back: 100 days before April 27 lands on January 17, 2026 — a Saturday. Useful for audit windows, warranty lookups, or review periods.

Frequently Asked Questions

What is the difference between adding days and adding months?▾
Adding days moves the date by an exact number of 24-hour periods — the result is always predictable regardless of which months you cross. Adding months shifts the calendar month by the specified count while keeping the day the same (subject to month-end clamping). For example, adding 30 days to January 31 gives March 2, but adding 1 month gives February 28. Use days when you need a precise duration; use months when you want the same calendar day in a future month.
How do leap years affect date calculations?▾
A leap year adds February 29 to the calendar, making the year 366 days instead of 365. This matters when you add a span of days that crosses a February 29 — the result will be one day later than in a non-leap year. It also matters when you add months: adding 1 month to January 31 in a leap year gives February 29, not February 28. Our calculator handles leap year logic automatically using the standard rule: a year is a leap year if it is divisible by 4, except century years (divisible by 100), which must also be divisible by 400.
How do business days differ from calendar days?▾
Calendar days count every day of the week including weekends and public holidays. Business days (also called working days) count only Monday through Friday, skipping Saturdays and Sundays. A 5-day calendar week contains 5 business days only if it spans Monday to Friday; a week starting on Wednesday contains only 3 business days (Wed, Thu, Fri). For contracts, shipping estimates, and legal deadlines, always clarify whether "days" means calendar days or business days.
What is the ISO 8601 date format?▾
ISO 8601 is the international standard for representing dates and times. The date format is YYYY-MM-DD — for example, April 27, 2026 is written as 2026-04-27. This format is unambiguous across cultures (unlike MM/DD/YYYY used in the US or DD/MM/YYYY used in much of Europe) and sorts correctly when treated as a string. ISO 8601 is widely used in databases, APIs, file names, and any system that needs to exchange dates reliably across regions.
How do I calculate the number of days until retirement?▾
Enter your target retirement date as the start date and today as the reference, then subtract to find the difference — or use our Days Between Dates calculator for a direct day count. For example, if you plan to retire on January 1, 2040 and today is April 27, 2026, the difference is approximately 5,000 calendar days (about 13 years and 8 months). Knowing this number helps with savings milestones, countdown motivation, and financial planning timelines.