Date Calculator

About This Date Calculator

A date calculator answers two everyday questions: “What date will it be a certain number of days, weeks, months, or years from now?” and “How much time separates two dates?” Instead of counting squares on a wall calendar — and risking an off-by-one error at a month boundary — this tool does the arithmetic instantly and shows the day of the week for every result.

What You Can Calculate

The Add / Subtract tab shifts any start date forward or backward by a number of days, weeks, months, or years. The Difference tab measures the gap between two dates three ways: a calendar breakdown in years, months, and days; the total day count; and a split of weekdays versus weekend days.

How to Use

1. On the Add / Subtract tab, pick a start date, choose Add or Subtract, enter an amount, and select a unit.
2. Click Calculate to see the resulting date and the weekday it falls on.
3. Switch to the Difference tab and enter two dates to see the years, months, days, total days, weekdays, and weekends between them.

How Date Arithmetic Works

Day and week calculations are fixed-length offsets, while month and year calculations follow calendar rules:

result date = start date + (amount × unit)

• Days: add or subtract the exact number of calendar days, crossing month and year boundaries as needed.
• Weeks: multiply the amount by 7 and shift by that many days — a week is always 7 days.
• Months: keep the same day-of-month and move forward or backward by the given number of months.
• Years: keep the same month and day and move by whole years (equivalent to 12 months per year).

Month-End Clamping

Months range from 28 to 31 days, so “the same day next month” does not always exist. When the target month is shorter, the result clamps to that month’s last day rather than spilling into the following month:

• January 31, 2026 + 1 month = February 28, 2026 — 2026 is not a leap year, so February has 28 days.
• January 31, 2024 + 1 month = February 29, 2024 — 2024 is a leap year, so February has 29 days.
• February 28, 2026 + 1 month = March 28, 2026 — no clamping, because March 28 exists.

Leap-Year Rules

A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2024 and 2028 are leap years, 2026 is not, 1900 was not, and 2000 was. Leap years add February 29, and this calculator accounts for that automatically in both directions — subtracting one month from March 31, 2026, for example, also lands on February 28, 2026.

Worked Examples

Adding 90 Days: March 1, 2026 → May 30, 2026

March contributes its remaining 30 days to reach March 31, April adds 30 days to reach April 30, and 30 more days lands on May 30. So March 1, 2026 + 90 days = May 30, 2026, a Saturday — the classic “net-90” payment deadline.

Subtracting 45 Days: June 15, 2026 → May 1, 2026

Counting back 14 days reaches June 1, and 31 more days covers all of May, so June 15, 2026 − 45 days = May 1, 2026, a Friday.

Adding 1 Month with Clamping

Starting from January 31, 2026 and adding one month lands on February 28, 2026 — not March 3 — because February 2026 has only 28 days and the result clamps to the month’s final day.

Days From a Date: Quick Reference

Common offsets counted from March 1, 2026 (a Sunday), exactly as this calculator computes them:

Note that 365 days from March 1, 2026 lands exactly one year later only because the span contains no February 29; across a leap day, 365 days and 1 year give different answers.

Calendar Days vs. Business Days

Everything above uses calendar days — every day on the calendar counts, including Saturdays and Sundays. Business days (Monday through Friday) skip weekends and, in stricter definitions, public holidays. The Difference tab helps here: it reports the total weekdays between two dates. For example, from March 1 to May 30, 2026 there are 90 elapsed days but only 65 weekdays — and 45 business days after March 1, 2026 is May 1, 2026. For dedicated counting, try our Day Counter, which shows days, weekdays, and weeks between any two dates. Neither tool excludes public holidays, so confirm statutory holidays for your jurisdiction when a deadline depends on them.

Practical Uses

• Deadlines and contracts: compute net-30, net-60, or net-90 payment due dates, notice periods, and contract expiration dates.
• Medication schedules: find when a 30-, 60-, or 90-day prescription runs out so you can refill on time.
• Return and warranty windows: check the last day of a 30-day return policy or a 1-year warranty from the purchase date.
• Project planning: set milestones by adding weeks to a kickoff date, then verify which weekday each milestone falls on.
• Travel and visas: verify “90 days within 180 days” stay limits and passport-validity windows.

Frequently Asked Questions