Date Calculator

Date calculations come up more often than most people realize.

How many days until your vacation? How many weeks between now and a project deadline? What date is 90 days from today for a contract term? How many business days are left in the quarter? This date calculator answers all of these questions instantly.

The tool offers two core functions. First, it calculates the number of days between any two dates, showing the result in days, weeks, months, and years. Second, it lets you add or subtract a specific number of days, weeks, months, or years from any starting date to find a target date. Both functions correctly handle leap years, varying month lengths, and calendar quirks that make manual date math error-prone.

Whether you are counting down to an event, calculating a deadline, determining how many days have passed since a milestone, or planning a timeline for a project, this tool removes the mental arithmetic and gives you precise results. For legal and business applications where "30 days" or "90 days" from a specific date matters, accuracy is not optional, and this calculator delivers it.

Calculating Days Between Two Dates

The most common use of a date calculator is finding the number of days between two dates. While this sounds straightforward, doing it manually is surprisingly error-prone because months have different lengths and leap years add an extra day to February every four years (with some exceptions).

How the calculation works. The tool counts every calendar day between the start date and the end date. If you enter January 1, 2025 and January 31, 2025, the result is 30 days (January 1 is day zero, January 31 is the 30th day after it). The count includes all intervening days regardless of weekends or holidays.

Including or excluding the end date. Depending on your purpose, you may want to include or exclude the end date in the count. "Days between" typically excludes both the start and end dates (counting only the days in between). "Days from" typically includes the end date. This calculator shows both counts so you can use whichever is appropriate for your situation.

Accounting for leap years. A leap year occurs every 4 years, except for years divisible by 100, unless also divisible by 400. So 2024 was a leap year, 2100 will not be, but 2000 was. February has 29 days in leap years and 28 in common years. The calculator handles this automatically for any date range.

Adding and Subtracting Days from a Date

The second core function answers the question: what date is X days from a given date?

Common scenarios:

• A contract states "payment due within 30 days of invoice date." If the invoice is dated March 5, payment is due April 4.
• A return policy says "returns accepted within 90 days of purchase." If you bought something on January 15, the deadline is April 15.
• A prescription is valid for 180 days from the date written. If written on June 1, it expires November 28.

Adding months. When adding months, the calculator moves to the same day number in the target month. Adding 1 month to January 31 yields February 28 (or February 29 in a leap year), since February does not have a 31st day. This is the standard convention used by most software systems and legal interpretations.

Subtracting days. The calculator also subtracts days. If you need to find out what date was 60 days before today, enter today’s date and subtract 60. This is useful for determining filing deadlines, statute of limitations dates, and lookback periods.

Business Days Calculator

Many legal, financial, and corporate deadlines are specified in business days (also called working days) rather than calendar days. Business days exclude weekends (Saturday and Sunday) and optionally exclude public holidays.

How business days are counted. In a typical week, there are 5 business days (Monday through Friday). A "10 business day" deadline from a Monday means the following Friday of the second week. If the starting day is a Wednesday, 10 business days ends on the Wednesday of the following week (skipping two weekends).

The holiday complication. Federal holidays, state holidays, and bank holidays vary by jurisdiction and can significantly affect business day calculations. In the US, there are typically 10 to 11 federal holidays per year, reducing the business days in a year from approximately 261 to about 250 to 251.

Business days per month. An average month has approximately 21 to 22 business days. The exact number varies by month and year depending on how weekends fall.

Important Date Ranges in Common Use

Certain day counts come up repeatedly in legal, financial, and administrative contexts.

30-day periods. Common in billing cycles, return policies, notice periods, and insurance grace periods. Note that "30 days" and "one month" are not the same thing, since months range from 28 to 31 days.

60-day periods. Used in some return policies, IRA rollover windows, and regulatory comment periods.

90-day periods. Standard for many legal notice requirements, probationary employment periods, visa validity windows, and financial reporting quarters.

180-day periods. Used for some insurance claim deadlines, COBRA continuation deadlines, and military deployment calculations.

365-day and 366-day periods. A full calendar year. Some contracts specify "365 days" regardless of leap years, while others specify "one year" which may include 366 days in a leap year. The distinction matters.

Working with Weeks and Months

While days are the most precise unit for date calculations, weeks and months are often more practical for planning and communication.

Weeks. One week is always exactly 7 days, making week calculations simpler than month calculations. There are approximately 52.177 weeks in a year. To convert days to weeks, divide by 7. The calculator shows results in weeks and remaining days (e.g., "45 days = 6 weeks and 3 days").

Months. Months are inherently imprecise as a time unit because they vary from 28 to 31 days. When the calculator reports a duration in months, it counts complete calendar months from the start date to the end date, plus remaining days. For example, from January 15 to March 20: January 15 to February 15 is one month, February 15 to March 15 is two months, plus 5 remaining days, for a total of 2 months and 5 days.

Calendar History and the Gregorian System

The calendar system used by this calculator (and most of the modern world) is the Gregorian calendar, introduced by Pope Gregory XIII in October 1582. It replaced the Julian calendar, which had been in use since 45 BC but accumulated an error of about 11 minutes per year, leading to a 10-day drift by the 16th century.

The Gregorian correction. When the Gregorian calendar was adopted, 10 days were skipped to realign the calendar with the solar year. In Catholic countries, the day after October 4, 1582 was October 15, 1582. Different countries adopted the Gregorian calendar at different times, with some not switching until the 20th century (Russia in 1918, Greece in 1923). Calculating dates across this transition period requires knowing which calendar was in use in the relevant location.

Leap year rules. The Gregorian calendar’s leap year rules (divisible by 4, except centuries, unless divisible by 400) produce an average year length of 365.2425 days, which is extremely close to the actual solar year of 365.2422 days. The remaining error amounts to about one day every 3,236 years.

Practical Date Calculation Tips

Count carefully around month boundaries. The most common errors in manual date math occur when a calculation crosses a month boundary, especially months with different lengths. Always verify with a tool rather than counting on your fingers.

Clarify calendar days vs business days. When someone says "within 10 days," confirm whether they mean calendar days or business days. The difference can be substantial: 10 calendar days from a Monday is the following Thursday, while 10 business days is the Monday after that.

Watch for inclusive vs exclusive counting. Some deadlines are "within 30 days" (exclusive of the start date), while others are "on or before the 30th day" (inclusive). Legal contexts often specify which convention applies, but in casual use, ambiguity is common. This calculator shows both counts.

For time-specific calculations (adding hours and minutes rather than days), use the Time Calculator. For workweek and timecard calculations, try the Hours Calculator.

Frequently Asked Questions

How do I calculate the number of days between two dates?

Enter your start date and end date into this calculator. It counts every calendar day between the two dates, accounting for varying month lengths and leap years. Results are shown in days, weeks, months, and years.

How do I find what date is 30 days from today?

Use the add/subtract function. Enter today’s date as the starting date, select "add," and enter 30 days. The calculator returns the exact date that is 30 calendar days from today.

Does the calculator account for leap years?

Yes. The calculator correctly handles leap years (years divisible by 4, except century years not divisible by 400). February has 29 days in leap years and 28 in common years, and all calculations reflect this.

What is the difference between calendar days and business days?

Calendar days include every day including weekends and holidays. Business days (working days) exclude Saturdays and Sundays, and optionally exclude public holidays. A 10 calendar day period is shorter than a 10 business day period.

How many days are in a year?

A common year has 365 days. A leap year has 366 days. The average Gregorian year is 365.2425 days. There are approximately 52 weeks and 1 day in a common year, or 52 weeks and 2 days in a leap year.

How do I calculate months between two dates?

The calculator counts complete calendar months between the start and end dates, plus remaining days. For example, from March 10 to June 25 is 3 months and 15 days (March 10 to June 10 is 3 months, plus 15 more days to June 25).

Can I subtract days from a date?

Yes. Enter a starting date, select "subtract," and enter the number of days. The calculator returns the date that is that many days before your starting date. This is useful for determining deadlines and lookback periods.

Calculations based on the Gregorian calendar system. Data accurate as of: March 2026