Measure date differences or add years, months, and days to a start date.
This tool provides estimates for informational purposes only and is not a substitute for professional advice. Individual results vary based on personal circumstances and assumptions.
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A date calculator is one of those tools that sounds simple but turns out to be remarkably useful once you start using it. How many days until your lease expires? How many weeks until a project deadline? What date is 90 days from today? What was the date 180 days ago? These are common questions in real life — for contracts, court deadlines, insurance renewals, pregnancy due dates, project management, travel planning, and dozens of other situations. This guide explains how date calculations work, how to use this tool, and answers the most common calendar-related questions people search for.
The simplest date calculation is finding the difference between two calendar dates — how many days from Date A to Date B. The mathematical process involves converting both dates to a common reference point (often called a Julian Day Number or a Unix timestamp in computers) and subtracting. Our calculator does this instantly, but it helps to understand what is happening under the hood.
Example: Days from January 1 to March 31 of the same year. January has 31 days, February has 28 (or 29 in a leap year), and March has 31 days. Total: 31 + 28 + 31 = 90 days (or 91 in a leap year). Our date difference calculator handles leap years, month-length variations, and all other edge cases automatically.
When you enter two dates and select "date difference" mode, you get the result in calendar days, approximate months, and approximate years. Calendar days is always exact; the months and years figures are approximations because months vary in length (28 to 31 days).
The second core function of this calculator is adding or subtracting a number of days, months, or years from a starting date to find the resulting date. This is useful for:
To use this function, select "add to date" mode, enter your starting date, then enter the number of years, months, and/or days you want to add. For subtraction, enter negative values or look for the subtract option. The result date accounts for all month-length and leap year rules automatically.
Many legal and professional deadlines are expressed in business days (also called working days) rather than calendar days. A business day is typically Monday through Friday, excluding public holidays. When someone says "we will respond within 5 business days," they mean 5 weekdays — so if you receive a notice on Thursday, the response period ends the following Thursday, not the following Tuesday.
In the USA, the 11 federal public holidays are: New Year's Day, Martin Luther King Jr. Day, Presidents' Day, Memorial Day, Juneteenth, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving Day, and Christmas Day. When counting business days, these must be excluded. State-specific holidays may also apply depending on your jurisdiction.
In England and Wales, there are typically 8 bank holidays per year: New Year's Day, Good Friday, Easter Monday, Early May Bank Holiday, Spring Bank Holiday, Summer Bank Holiday, Christmas Day, and Boxing Day. Scotland and Northern Ireland have additional and different bank holidays. Like the US, UK contracts and court deadlines expressed in "working days" exclude bank holidays.
Contract law in both the USA and UK frequently uses date ranges. A 30-day notice period means the recipient has 30 calendar days from the date the notice is received (usually). Court filing deadlines, statutory response periods, and appeal windows are all date-specific. Getting the date wrong can have serious consequences — this calculator gives you a quick, reliable answer.
The standard way to calculate a pregnancy due date is to add 280 days (40 weeks) from the first day of the last menstrual period (LMP). Alternatively, add 266 days from the confirmed date of conception. Use our Due Date Calculator for a pregnancy-specific version with trimesters and milestone dates.
The number of days from your date of birth to today gives your exact age in days. Divide by 365.25 (to account for leap years) to get your age in years. Our Age Calculator provides your exact age in years, months, and days in one click.
Calculating interest periods, bond maturity dates, CD maturity dates, and investment holding periods all require accurate date arithmetic. A 91-day Treasury bill started on March 1 matures on May 31, for example.
Sprints, deliverables, launch dates, and review cycles all depend on date math. If a project starts on a given date and runs for 12 weeks (84 calendar days), when does it end? If you have 3 weeks of buffer before a hard deadline, what is the last possible start date? This calculator answers those questions instantly.
The Gregorian calendar is the international standard and the one this calculator uses. It replaced the older Julian calendar in the 16th century and corrected a small but accumulating drift in the Julian system. Key facts about the Gregorian calendar:
This is one of the most common date-related questions. Here is the complete reference:
The classic mnemonic: "30 days hath September, April, June, and November; all the rest have 31, excepting February alone, which hath 28 days clear, and 29 each leap year."
A leap year adds an extra day to February (making it 29 days) to keep the calendar aligned with Earth's orbit around the sun. The solar year is approximately 365.242 days — so every four years we add a day to compensate. The Gregorian rule: divisible by 4 = leap year, except centuries which must be divisible by 400.
Upcoming leap years: 2024, 2028, 2032, 2036, 2040. The next century that qualifies: 2400.
This matters for date calculations because adding "one year" to February 29 is ambiguous — people born on Feb 29 typically celebrate on Feb 28 or March 1 in non-leap years, depending on the country and context.
To convert a day count to weeks, divide by 7. For example, 98 days is 14 weeks exactly. 100 days is 14 weeks and 2 days. For project planning in two-week sprints, knowing the number of complete weeks in a date range is particularly useful.
Reference table:
Enter today's date in the start field, select "add to date" mode, enter 30 in the days field, and the calculator will show you the exact result date including any month-length adjustments.
Enter today's date, select "add to date" mode, and enter -90 days (or a subtract option if available). This is commonly needed for statute of limitations questions, insurance claim windows, and warranty periods.
Enter today's date and December 25 of the current or next year. Divide the resulting days by 7. Our calculator shows the approximate months count as well as the exact day count.
A standard calendar year has 365 days. A leap year has 366 days. The tropical year (Earth's actual orbit) is approximately 365.2422 days, which is why we need leap years.
Count the total days in each month covered by the date range. Add partial months at the start and end. Adjust for leap years if February is included. This calculator does it instantly and accurately.
This calculator returns calendar days by default. Calendar days include weekends and public holidays. For business day calculations, you would need to subtract weekends and applicable holidays from the result.
Yes. The calculator shows an approximate months count alongside the exact days. Note that months are an approximation since they vary in length — the exact day count is always more precise for deadline calculations.
The calculator uses standard JavaScript date objects and can handle dates across many centuries. For historical or future dates beyond common ranges, verify results independently.
Disclaimer: This date calculator is for informational and planning purposes. For legal or contractual deadline calculations, verify results with a legal professional and consult applicable laws in your jurisdiction.