Date Calculator – Add or Subtract Days, Weeks & Months from a Date

Find past or future dates instantly by adding or subtracting time from any date. Works with days, weeks, months, and years for deadlines, events, and planning.

Start Date

Operation

Amount

Unit

How to Use This Date Calculator

Select your starting date

Choose any date from the calendar picker. This is your reference point for adding or subtracting time.

Choose add or subtract and enter the amount

Select whether to move forward or backward in time. Enter the number of days, weeks, months, or years to calculate.

Click Calculate to see the result

The calculator handles month boundaries, leap years, and varying month lengths automatically.

Date Calculation Reference

Unit	Equals	Notes
1 week	7 days	Fixed length
1 month	28-31 days	Varies by month
1 year	365 or 366 days	366 in leap years
Average month	30.44 days	365.25 / 12
Leap year	Every 4 years	Except century years not divisible by 400

Month calculations preserve the day number when possible. Adding 1 month to January 31 gives February 28 (or 29 in leap years).

Understanding Date Arithmetic

Why Month Addition Is Tricky

Months have different lengths: 28, 29, 30, or 31 days. When you add "1 month" to January 31, should the result be March 3 (31 days later) or February 28 (same day number)? This calculator uses day-number preservation — the result is February 28.

Leap Year Rules

Leap years occur every 4 years, adding February 29. Exception: century years (1900, 2000) are only leap years if divisible by 400. So 2000 was a leap year, but 1900 was not. This keeps our calendar aligned with Earth's orbit.

Business Days vs Calendar Days

This calculator uses calendar days. For business calculations, you'd exclude weekends and holidays. Adding 30 business days typically equals about 6 weeks of calendar time. Use a business day calculator for project deadlines.

ISO 8601 Date Format

Dates are displayed in YYYY-MM-DD format (ISO 8601). This international standard avoids confusion between MM/DD/YYYY and DD/MM/YYYY formats. It also sorts correctly in alphabetical order.

Common Use Cases

Project Deadlines

Calculate when a project is due by adding the timeline to the start date. Add 90 days for a quarterly project, or 6 months for a longer initiative.

Contract End Dates

Find when a lease, subscription, or employment contract ends. Add 12 months for annual contracts or 36 months for typical car leases.

Pregnancy Due Dates

Calculate estimated due dates by adding 40 weeks (280 days) to the last menstrual period. This is Naegele's rule, used by healthcare providers.

Historical Research

Subtract years to find dates in the past. Calculate ages, anniversaries, or how long ago historical events occurred.

Frequently Asked Questions

How do you add months to a date?

Add the number of months to the month component, keeping the same day number. If the resulting month doesn't have that day (like February 31), use the last day of that month instead.

Does this calculator account for leap years?

Yes. When adding years or calculating across February, the calculator correctly handles leap years. February 29, 2024 plus 1 year equals February 28, 2025 (no Feb 29 in 2025).

What's the difference between adding 30 days vs 1 month?

Adding 30 days always moves forward exactly 30 calendar days. Adding 1 month moves to the same day number next month, which could be 28, 29, 30, or 31 days depending on the months involved.

Can I calculate dates in the past?

Yes. Use the "Subtract" operation to go backward in time. This is useful for finding historical dates, calculating birth dates from ages, or determining when something started.

How accurate is this for business planning?

For calendar-based planning, it's exact. For business-day planning (excluding weekends/holidays), use a dedicated business day calculator. Bank processing, shipping estimates, and project timelines often need business days, not calendar days.

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