Date Calculator

What is Date Calculator?

A date calculator performs arithmetic with calendar dates — finding the duration between two dates or calculating a future/past date by adding or subtracting time periods. Unlike simple number math, date calculations must account for months with different lengths (28, 29, 30, or 31 days), leap years, and the distinction between calendar days and business days. Our calculator has two modes: "Days Between Two Dates" shows the exact duration in years, months, days, total days, weeks, hours, and minutes. "Add or Subtract from a Date" lets you add or subtract any combination of years, months, weeks, and days — with an option for business days only.

How It Works

In "Between" mode, enter a start and end date. The calculator finds the difference by counting complete years, then remaining months, then remaining days — properly handling month-length variations and leap years. Total days are calculated using millisecond timestamps for exact accuracy. In "Add/Subtract" mode, enter a base date and the time to add or subtract. For business days, the calculator skips weekends when counting.

Formula

Days between = (endDate − startDate) in milliseconds / 86,400,000
Business days: count weekdays (Mon-Fri) in range
Add months: date.setMonth(date.getMonth() + n)
Add days: date.setDate(date.getDate() + n)

Formula Explained

JavaScript Date objects store time as milliseconds since January 1, 1970 UTC. Dividing the millisecond difference by 86,400,000 (milliseconds per day) gives exact total days. For the human-readable breakdown (years, months, days), we count hierarchically — full years first, then remaining full months, then remaining days — adjusting for month boundaries.

Example

Between January 1, 2025 and October 15, 2025: 9 months, 14 days = 287 days = 41 weeks 0 days Business days: ~205 (excluding weekends) Add 90 days to March 1, 2025 = May 30, 2025 Subtract 2 months from December 31, 2025 = October 31, 2025

Tips & Best Practices

✓Use business days for project planning and deadline calculations.
✓Remember that "days between" by default excludes the end date — check "include end day" if you want both dates counted.
✓For pregnancy due dates, add 280 days (40 weeks) to the first day of the last period.
✓Notice periods and contract terms often specify calendar days vs business days — know which one applies.

Common Use Cases

•Calculating project timelines and deadlines
•Finding the exact number of days until an event
•Planning notice periods for employment or contracts
•Determining age in exact days for legal or medical purposes
•Scheduling recurring events with specific intervals

Frequently Asked Questions

Subtract the earlier date from the later date. Our calculator handles leap years, varying month lengths, and shows the result in years, months, days, total days, weeks, hours, and minutes. You can optionally include the end day in the count.

Business days (or working days) exclude weekends (Saturday and Sunday). Our calculator can count business days between dates or add/subtract business days from a date. Note: it does not account for public holidays, which vary by country and region.

Yes. The calculator correctly handles leap years (every 4 years, except centuries not divisible by 400). February 29 is properly counted when it falls within the date range.

Switch to "Add or Subtract" mode, enter your start date, and specify the number of months. The calculator handles end-of-month edge cases — for example, adding 1 month to January 31 gives February 28 (or 29 in a leap year).

Yes. In "Add or Subtract" mode, click the Subtract button and enter the number of years, months, weeks, or days to go backward from your start date.