About This Tool
Counting days between two dates by hand is tedious and error-prone. Calendar months have different lengths, leap years add an extra day to February, and figuring out how many business days fall within a range requires checking every single day against the weekend. This Date Calculator handles all of that instantly. It operates in two modes: "Days Between Dates" calculates the complete difference between any two dates in days, weeks, months, years, and business days; "Add/Subtract Days" takes a starting date and a number of days to produce the resulting calendar date. Use the first mode when you need to know how long a project lasted, how many days remain until a deadline, or how many working days fall within a contract period. Business-day counts exclude Saturdays and Sundays, giving you the number of actual workdays in the range. Use the second mode when a deadline is described as "90 days from today" or "60 business days from signing" and you need the exact calendar date. The result appears in multiple formats — MM/DD/YYYY, ISO (YYYY-MM-DD), and a written-out long form — so you can paste the format you need directly into documents, emails, or spreadsheets. Your data stays private and nothing is stored or shared.
How the Days-Between Calculation Works
The calculator converts both dates to a common reference point (milliseconds since the epoch) and divides the absolute difference by the number of milliseconds in one day (86,400,000). This approach automatically accounts for leap years, daylight saving time transitions, and varying month lengths. The result is always a whole number of calendar days.
Weeks are derived by dividing total days by 7. Months are computed by counting how many full calendar months fit between the two dates, factoring in the day-of-month to avoid overcounting partial months. Years follow the same logic, requiring the later date to have reached or passed the anniversary of the earlier date within the final year.
Business Days Explained
Business days (also called working days or weekdays) exclude Saturdays and Sundays. The calculator iterates through each day in the range and counts only those falling on Monday through Friday. This metric is crucial in several contexts:
- Legal deadlines: Court filing periods, regulatory response windows, and statutory notice periods are often specified in business days.
- Shipping estimates: Carriers quote delivery times in business days because they do not operate on weekends.
- Financial settlements: Stock trades settle in T+2 business days, and wire transfers process only on banking days.
- Project management: Sprint planning and resource allocation are based on available working days, not calendar days.
Note that this calculation does not account for public holidays, which vary by country and region. For holiday-adjusted business day counts, subtract the applicable holidays manually.
Adding and Subtracting Days
The add/subtract mode starts with your chosen date and moves forward or backward by the specified number of days. This is useful for calculating due dates, expiration dates, and planning deadlines. Common use cases include:
- 90-day warranty periods: Enter the purchase date and add 90 to find the warranty expiration.
- 30-day return windows: Add 30 days to the delivery date to determine the last day you can return an item.
- Medication schedules: A 14-day course of antibiotics starting on a specific date — when does it end?
- Countdown planning: Subtract days from an event date to set milestone reminders.
Date Formats and Parsing
This calculator accepts multiple date formats in the text input field:
- MM/DD/YYYY — the most common format in the United States (e.g., 03/15/2024)
- YYYY-MM-DD — the ISO 8601 international standard (e.g., 2024-03-15)
- Month Day, Year — written form (e.g., March 15, 2024)
- Day Month Year — common in Europe and Asia (e.g., 15 March 2024)
Enter the date in whichever format is most natural to you. The calculator parses it and normalizes it internally for accurate computation.
Practical Applications
Date arithmetic shows up in more places than most people realize. Human resources departments calculate employee tenure in years and months. Real estate closings require counting days for inspection periods, contingency deadlines, and mortgage rate locks. Academic institutions set semester start and end dates based on a required number of instructional days. Tax filing deadlines are often stated as a specific number of days after the end of a fiscal period. Even personal planning — tracking pregnancy due dates, vacation countdowns, or retirement dates — benefits from precise date math.
Frequently Asked Questions
Does the business-day count include holidays?
No. The business-day calculation excludes only Saturdays and Sundays. Public holidays vary by country, state, and employer, so they are not automatically removed. To get a holiday-adjusted count, subtract the number of applicable holidays that fall on weekdays within the date range from the business-day result.
How does the calculator handle leap years?
Leap years are handled automatically. The underlying date system knows that years divisible by 4 (except centuries not divisible by 400) have 366 days. February 29 is included in all day counts when it falls within the selected range.
Can I calculate negative date differences?
The "Days Between Dates" mode always returns a positive result regardless of which date is entered first. The calculator takes the absolute difference. In the "Add/Subtract Days" mode, you can explicitly subtract days to move backward in time.
What is the maximum date range supported?
The calculator supports dates from the year 0001 to 9999. For most practical purposes — historical research, future planning, or contract calculations — this range is more than sufficient. Dates outside this range may produce unexpected results due to calendar system limitations.
How are months calculated between dates?
A "month" is counted as a complete calendar month from one date to the corresponding date in the next month. If the start date is January 15 and the end date is March 10, the result is 1 complete month (January 15 to February 15) plus 23 additional days — so the month count is 1. This matches the common-sense understanding of "full months elapsed."
What date formats can I use?
- MM/DD/YYYY — e.g., 06/15/2024
- YYYY-MM-DD — e.g., 2024-06-15 (ISO format)
- Written out — e.g., June 15, 2024
Any format that is commonly recognized as a valid date string will be parsed correctly.