Date Calculator: Add or Subtract Days, Weeks, Months & Years

How to Use Our Date Calculator

Our tool is designed for maximum flexibility, allowing you to perform two main types of calculations: finding a future/past date or measuring the duration between two dates.

1. To Add or Subtract Time from a Date

Use this function to find a target date. For example, “What date is 90 days from today?” or “What was the date 6 months ago?”

• Start Date: Select the date you want to begin your calculation from. It defaults to today’s date, August 3, 2025, but you can change it to any date.

• Operation: Choose whether you want to Add (+) or Subtract (-) time from your start date.

• Years / Months / Weeks / Days: Enter the amount of time you wish to add or subtract. You can use any combination of these fields (e.g., add 1 year and 15 days).

2. To Calculate the Duration Between Two Dates

Use this function to find the total amount of time that has passed between a start and end point. For example, “How many days were there between March 5, 2023, and today?”

• Start Date: Enter the beginning date of the period you want to measure.

• End Date: Enter the final date of the period. The calculator will automatically compute the time between them.

Understanding Your Results

The calculator’s output is simple and direct, but it’s important to know exactly what it means. All calculations correctly account for the different number of days in each month and for leap years.

Result for Adding/Subtracting Time

When you add or subtract a duration, the result is the Target Date. This is the exact calendar date you land on after the calculation is complete.

Example Calculation: You have an invoice dated August 3, 2025, with payment terms of “Net 60” (due in 60 days).

Result for Calculating Duration

When you find the time between two dates, the result is a precise Time Span. We provide this in two useful formats:

1. Total Days: A single number representing the total count of full days between the two dates.

2. Years, Months, Days Breakdown: The same duration, but broken down into a more common format (e.g., “2 years, 4 months, and 18 days”).

Common Time Durations and Their Uses

Certain time spans are used so frequently in business, legal, and personal contexts that they have become standard. Here are a few common ones you can calculate instantly.

Frequently Asked Questions

How does the calculator handle leap years?

Our calculator’s algorithm automatically identifies leap years (like 2024, 2028, 2032) and includes February 29th in its calculations. This ensures that when you add or subtract time spanning a leap year, the resulting date is completely accurate. You don’t need to do any manual adjustments.

What is the difference between adding “30 days” and adding “1 month”?

This is a critical distinction that often causes confusion. The results can be different depending on the start date.

• Adding “30 days” is a fixed duration. The calculator counts forward exactly 30 days.

• Adding “1 month” moves to the same date in the following month. The number of days this represents depends on the length of the starting month.

Concrete Example:

• February 1, 2026 + 1 month = March 1, 2026 (A duration of 28 days)

• March 1, 2026 + 1 month = April 1, 2026 (A duration of 31 days)

For precise time intervals, using “days” or “weeks” is often best. For monthly anniversaries (like payment due dates), use “months”.

Can I use this to calculate only business days or exclude weekends?

This tool calculates based on calendar days, meaning it includes every day of the week (Monday-Sunday) and public holidays. It is designed for calculating absolute time spans.

For timelines that depend on a 5-day work week, you would need a specialized tool. For calculations that need to exclude weekends and holidays, our dedicated [Business Day Calculator] is the perfect tool.

How do I calculate a pregnancy due date?

A standard full-term human pregnancy is estimated to be 40 weeks. To calculate an estimated due date, you can use our calculator with the date of the mother’s last menstrual period (LMP).

Concrete Example:

• Start Date: Enter the first day of the last menstrual period (e.g., June 14, 2025).

• Operation: Select Add (+).

• Weeks: Enter 40.

• Result (Estimated Due Date): March 21, 2026.

How do I use this for financial billing cycles (e.g., Net 30, Net 60)?

This calculator is perfect for managing invoice due dates.

1. Set the Start Date to your invoice date.

2. Select the Add (+) operation.

3. In the Days field, enter the number specified by your payment terms (e.g., 30, 60, or 90).

4. The resulting date is your payment due date.

When calculating duration, is the end date included in the count?

Our calculator follows the most common standard: it measures the number of full 24-hour periods between the start and end dates. Therefore, the total count does not include the end date itself.

Concrete Example:

• Start Date: August 4, 2025 (Monday)

• End Date: August 6, 2025 (Wednesday)

• Result: The duration is 2 days. (The full day of Aug 4-5 and the full day of Aug 5-6).

Can I work with historical dates with this tool?

Yes. The calculator uses the Gregorian calendar system and can accurately compute dates and durations for events far in the past. This is useful for historical research, academic projects, or simple curiosity.

Concrete Example: Calculate the duration of the American Civil War.

• Start Date: April 12, 1861 (Battle of Fort Sumter)

• End Date: April 9, 1865 (Lee’s Surrender at Appomattox)

• Result: 3 years, 11 months, and 28 days.

What calendar system does this calculator use?

The calculator operates on the Gregorian calendar, which is the internationally accepted civil calendar used by the vast majority of the world. It was introduced in October 1582 as a refinement to the Julian calendar to more accurately reflect the length of a solar year. All calculations, including leap years, are based on its rules.

How can I calculate my exact age with this tool?

You can easily find your age by using the “duration between two dates” function.

1. In the Start Date field, enter your date of birth.

2. In the End Date field, leave it as today’s date (or enter any other date you want to know your age on).

3. The result will show your precise age in years, months, and days.

Besides deadlines, what are other common uses for this calculator?

This tool is incredibly versatile. People use it for:

• Project Management: Setting milestones and final delivery dates.

• Personal Planning: Counting down to vacations, weddings, or other events.

• Legal & Contracts: Determining contract start/end dates, warranty periods, and statutes of limitations.

• Health & Fitness: Tracking medication schedules or planning fitness programs over a set number of weeks.

• Cooking & Food Safety: Calculating “use by” dates for homemade goods.

Take the Next Step

Now that you’ve mastered date calculations, you can apply them to other areas of your life.

• Now that you’ve found a key date, see exactly how old you’ll be on that day with our Age Calculator.

• If your date calculation is for a loan or financing, estimate your repayment schedule with our Loan Amortization Calculator.

• If you’re planning a project timeline, ensure it fits within your budget by tracking expenses with our Budget Calculator.