Date Calculator

Find the number of days between any two dates, add or subtract days from a date, and see the weekday of any date — with business days counted too. Free, instant, and private: everything runs in your browser.

How to Use the Date Calculator

For the gap between two dates, pick a start date and an end date and press Calculate Days. The result shows the total days between them, the same gap in weeks and in calendar months and days, how many of those days are business days (Monday to Friday), and the weekday each date falls on. The start date itself counts as day zero and the end date is the final day counted — so two dates one day apart give 1 day, and the same date twice gives 0 days.

To move forward or backward from a date, use the second calculator: pick a start date, enter the number of days, choose Add or Subtract, and press Calculate Date. It shows the resulting date, its weekday, and how far that date is from today — handy for deadlines ("90 days from now"), anniversaries, and counting back from a fixed date ("100 days before the launch"). Both calculators work with past, present and future dates, and nothing you enter leaves your browser.

How Date Math Works

Date math is calendar subtraction, not division. The total days between two dates is simply the end date minus the start date, counted in real calendar days — which is why months of different lengths and leap years need no special handling: the calendar already contains them. A year is a leap year when it is divisible by 4, except century years, which must be divisible by 400 — so 2024 was a leap year, 2100 will not be, and 2000 was.

The months-and-days breakdown walks the calendar the same way: full years first, then full months, then the remaining days, borrowing days from the real previous month when the day count goes negative. Business days are counted separately by stepping through each date in the range and keeping only Mondays to Fridays. Time zones and daylight saving are ignored — the tool counts midnight-to-midnight local days, which is how people mean dates in everyday use.

Worked Example

Start date 1 January 2026, end date 12 November 2026:

Step 3 shows the calendar walk: January to November is 10 full months, and the 12th minus the 1st leaves 11 days. Step 5 is the one people usually estimate wrong — an 11-calendar-day stretch that starts on a Monday holds only 9 business days, because the two weekend days in the middle do not count. Step 8 is the leap-year check: the same pair of dates in 2025 or 2026 is only 1 day, because there is no 29 February to count.

Date Math People Get Wrong

The most common mistake is the off-by-one: counting both end dates. If a project runs from 1 November to 30 November, that is 29 days between the dates, not 30 — the 1st is day zero. If you need an inclusive count (both dates counted), add 1 to the total. The second mistake is assuming every month has 30 days: dividing total days by 30 gives a months figure that is wrong by days, and dividing by 365.25 does the same to years. Real months are 28 to 31 days long, and only the calendar walk gets them right.

The third mistake is forgetting leap day. A span that crosses 29 February gains a day that a non-leap span of the same dates does not have, which quietly shifts anniversaries, interest periods and deadlines by one. The fourth is treating business days like calendar days: a "10-day" turnaround quoted on a Friday is really two full weeks on the calendar. And the fifth is time zones — a timestamp converted across zones can land on a different date, which is why this tool works in whole local days and never in hours. If your dates came from a system that stores timestamps, convert them to plain dates first with our Unix Timestamp Converter.

Limitations

The calculator uses the proleptic Gregorian calendar, the standard civil calendar used worldwide — dates before 1582 are mathematically extended backwards and have no historical meaning. Business days are Monday to Friday only: public holidays are not excluded, because no single holiday list fits every country or US state, so check holidays separately for deadline work. The tool counts whole days only — it cannot split a day into hours, so for time-of-day gaps use the Time Duration Calculator. Results are date arithmetic for planning, not legal advice: contract and court deadlines follow jurisdiction-specific counting rules that can differ from the plain calendar count shown here.

A date gap settles more than curiosity — it is the number behind project plans, pregnancy weeks, visa stays and savings goals. It pairs naturally with the Age Calculator when the gap starts at a birth date, and with the Pregnancy Due Date Calculator when the dates are pregnancy weeks. For the hours inside a single day, the Time Duration Calculator finishes the picture.

Frequently Asked Questions

Guides for This Tool

Related Tools