Date Calculator

How to use this tool

Select a Start Date, choose whether to Add (+) or Subtract (−) time, enter one non-negative whole-number amount, select Days, Weeks, Months, or Years, then select Calculate. Apply separate calculations deliberately when combining units.

If you already have two calendar dates and want to count the exact days, weeks, and weekends between them, use the Days Between Dates Calculator.

If your deadline counts Monday-through-Friday working days and can exclude holiday dates you enter manually, use the Business Days Calculator.

If you want to find a person's exact chronological age in years, months, and days, use the Age Calculator.

Transparent method: how calendar date addition and subtraction work

Adding or subtracting calendar periods is not the same as multiplying by a fixed 30-day month. Under the Gregorian calendar rules documented by the U.S. Naval Observatory and NASA's ISO 8601 Date and Time Reference, the calculator applies the single selected unit for each calculation:

Months and years use calendar-aware clamping: Adding 1 month to January 15 lands on February 15. When you add 1 month to the end of a 31-day month—such as January 31 + 1 month—the calculator clamps the result to the final valid day of the target month (February 28 in a common year, or February 29 in a leap year such as 2024 or 2028).

Weeks and days use exact 7-day and 1-day steps: Each week adds or subtracts exactly 7 calendar days (Weeks × 7), and each day steps forward or backward across month and year boundaries using exact month lengths (28, 29, 30, or 31 days).

Quick-reference table: 30, 45, 60, 90, 120, and 180 days vs. calendar months

Contract notices, visa stays, prescription refills, and warranty windows often specify "90 days" rather than "3 calendar months"—and the two rules frequently land on different calendar dates:

How to add or subtract dates in Microsoft Excel and Google Sheets

If you are tracking renewal dates or project milestones in a spreadsheet (with your Start Date in cell A2), Microsoft's Date and Time Functions Reference and EDATE documentation provide four clean formulas:

3 common date-calculation mistakes to avoid

Treating "90 days" and "3 months" as interchangeable: Seven months of the year have 31 days. Any three-month stretch between March and October contains 91 or 92 calendar days, not 90. If a passport validity rule, probation clause, or warranty states "90 days," always add 90 days rather than 3 months.

Forgetting end-of-month clamping when chaining calculations: Because January 31 + 1 month clamps to February 28, adding another 1 month to February 28 yields March 28 (whereas adding 2 months directly to January 31 yields March 31). Always add the full month count directly to your original anchor date.

Mixing US (MM/DD/YYYY) and UK/International (DD/MM/YYYY) date formats: Entering 05/11/2026 can mean May 11 in MM/DD/YYYY notation or November 5 in DD/MM/YYYY notation. Use the unambiguous ISO format (YYYY-MM-DD, such as 2026-11-05) whenever recording contract deadlines.

Spreadsheet DATE normalizes overflowing days; it does not always clamp like this calculator. Use EDATE for month offsets when you want end-of-month clamping. Calendar arithmetic does not determine medical, visa, tax or contractual deadlines.

Assumptions and limits

• The amount must be a non-negative whole number; the Add or Subtract control sets the direction.
• One week is exactly seven calendar days.
• End-of-month dates are clamped rather than rolled into the following month.
• A February 29 date moved to a non-leap year becomes February 28.

Sources

• Leap Years — Astronomical Applications Department
• International Standard Date and Time Notation (ISO 8601)
• EDATE Function — Add or Subtract Months from a Date