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Our free date calculator answers two everyday questions in seconds: how many days are between two dates, and what date falls a certain number of days from today. Great for deadlines, anniversaries, and travel planning.
How many days until your vacation? What date is 90 days from today? If you have ever tried to answer these questions by counting on a calendar, you know that date math is deceptively hard. Months refuse to cooperate — they run 28, 29, 30, or 31 days — leap years sneak an extra day into February every four years, and innocent phrases like "three months from now" turn out to be ambiguous. The free date calculator on this page does the hard work for you. It answers two everyday questions in seconds: how many days fall between any two dates, and what date results from adding or subtracting a number of days from a starting date. Below the calculator you will find a complete guide — how to use the tool, what date calculations actually mean, the exact formulas with worked examples, two practical walkthroughs with real numbers, the five mistakes almost everyone makes, related tools, and answers to the most common questions.
The calculator above has two modes. Days Between mode answers "how many days is that?" — the length of a trip, the days until a deadline, the duration of a project, or the gap between any two calendar dates. Add/Subtract mode answers "what date will it be?" — add days to find a future date (90 days after January 10 is April 10), or subtract days to find a past date (30 days before a deadline).
To find the days between two dates, follow these steps:
To add or subtract days, follow these steps:
A few tips. First, leap years are handled automatically: date ranges that include February 29 get the extra day without any special input. Second, results show the weekday of the resulting date, so you can immediately see whether a deadline lands on a weekend. Third, everything runs privately in your browser — no dates are saved or sent anywhere.
Date calculations are arithmetic performed on calendar dates: finding the number of days between two dates, or shifting a date forward or backward by a number of days. They look simple, but the irregular calendar makes mental math unreliable — which is why understanding the building blocks helps.
A calendar date identifies one specific day on the Gregorian calendar: a year, a month, and a day-of-month, such as October 8, 2026. The Gregorian calendar, introduced in 1582 and now the international civil standard, is the system behind every date you enter on this page. Its months are irregular by design — a historical accident we all live with: January, March, May, July, August, October, and December have 31 days; April, June, September, and November have 30; February has 28, or 29 in leap years. The old knuckle mnemonic still works: months on knuckles have 31 days, months in the valleys have 30 (or 28).
A Julian day (more precisely, the Julian Day Number) is the astronomers' solution to calendar irregularity: a continuous count of days since noon on January 1, 4713 BC. Under this system, every date becomes a single number, and the days between two dates is just subtraction. October 8, 2026 is Julian Day 2,461,321. You never need to compute these numbers by hand — the calculator does it behind the scenes — but the concept explains why software finds date math trivial while humans find it error-prone.
A leap year is the calendar's correction for the fact that Earth orbits the Sun in about 365.2422 days, not exactly 365. The rule: a year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400. So 2024 was a leap year, 2028 will be next, 2000 was a leap year, but 1900 and 2100 are not. Leap years add February 29, and any date calculation spanning that day must count it — one reason manual counting goes wrong.
Date duration is the number of days separating two dates — the core output of this calculator. From March 15 to July 4, 2026, the date duration is 111 days. Durations are always counted in whole days; this tool follows the standard convention of exclusive counting (the start day itself is not counted, as explained in Common Mistakes below).
Business days are the working-day subset of calendar days: Monday through Friday, excluding public holidays. A year has about 261 business days versus 365 calendar days. Contracts, shipping estimates, and legal deadlines usually run in business days, while rent, subscriptions, and visas run in calendar days — mixing them up is one of the classic errors covered below. As a rule of thumb, 30 business days span about 42 calendar days (six full weeks).
Why is February short? Blame the Romans: the early Roman calendar began in March, leaving February as the last month, where the leftover days (and later the leap-day correction) were stuffed. The Gregorian reform of 1582 then deleted 10 days to realign the calendar with the seasons — October 4, 1582 was followed directly by October 15. Modern date software, including this calculator, uses the proleptic Gregorian calendar, extending today's rules backward and forward uniformly.
Every calculation on this page reduces to one idea: convert each date to a day number, then add or subtract. The days-between formula is: Days Between = DayNumber(End Date) − DayNumber(Start Date), where DayNumber is the Julian Day Number (days since January 1, 4713 BC). The add/subtract formula is: Result Date = DayNumber(Start Date) ± N, converted back to a calendar date. Because the day numbers already encode month lengths and leap years, no special cases are needed.
If you want to verify by hand, use the month-by-month method: count the remaining days in the start month, add the full months in between, then add the days in the end month. The leap-year rule (divisible by 4, except centuries must be divisible by 400) tells you whether February contributes 28 or 29 days.
Worked example 1 — days between two dates. Start: March 15, 2026. End: July 4, 2026. Is 2026 a leap year? 2026 ÷ 4 = 506.5 — not divisible, so February has 28 days (irrelevant here anyway). Count: March has 31 days, so remaining in March = 31 − 15 = 16 days. Full months: April = 30, May = 31, June = 30. Days in July before the end date = 4 days. Total: 16 + 30 + 31 + 30 + 4 = 111 days. As weeks: 111 ÷ 7 = 15.86 weeks (15 full weeks and 6 days).
Worked example 2 — adding days to a date. Start: January 10, 2026. Add 90 days. January has 31 days, so remaining in January = 31 − 10 = 21 days; 90 − 21 = 69 left. February 2026 has 28 days (not a leap year); 69 − 28 = 41 left. March has 31 days; 41 − 31 = 10 left. April absorbs the final 10 days, landing on April 10, 2026. Notice how the irregular months make this impossible to do as "3 months" — February's shortfall is exactly why day-counting beats month-counting.
You arrive in a country on January 10, 2026, and your visa allows a 90-day stay. What is the last legal day? Enter January 10, 2026, type 90, choose Add, and the calculator returns April 10, 2026 — verified by the hand calculation above (21 + 28 + 31 + 10 = 90). The weekday is shown too, so you can plan travel around it. The critical lesson: never approximate this as "three months." January 10 plus three calendar months is also April 10 here, but only by coincidence of the specific months — try the same trick starting February 10 (90 days → May 11, while "3 months" suggests May 10) and the approximation breaks. Border officials count days, not months, and overstaying by even one day can mean fines or entry bans.
Your client signs off on October 6, 2026 (a Tuesday), and the contract gives you 30 days. When is the deadline? Enter October 6, 2026, type 30, choose Add: October has 31 days, so 31 − 6 = 25 remaining; 30 − 25 = 5 days into November → November 5, 2026. The calculator also reports the weekday — Thursday — so you know the deadline falls on a workday, not a weekend. If the contract had specified 30 business days instead, the answer would be roughly six weeks later (around November 17), which is why the calendar-day versus business-day distinction in the contract matters enormously.
Date math has five classic traps. Here they are, with fixes.
Dates rarely travel alone — these companion tools cover the neighboring questions. The Age Calculator computes an exact age in years, months, and days from any birth date, handling leap years automatically — perfect for birthdays, anniversaries, and eligibility checks. The Countdown Timer counts down live to any future date and time in days, hours, minutes, and seconds, ideal for launches, exams, and holidays. And when your question is about clock times rather than calendar dates, the Time Calculator measures durations between times and adds or subtracts hours and minutes.
This page provides a free Date Calculator at https://sigmacalculator.net/time-date-calculators/date-calculator/, operated by SigmaCalculator. Function: compute the number of days between two calendar dates (Days = JulianDayNumber(end) − JulianDayNumber(start)) and add or subtract N days from a date, reporting the resulting date and weekday. Inputs: start date, end date, or start date plus day count, on the proleptic Gregorian calendar with automatic leap-year handling (divisible by 4, except centuries divisible by 400). Outputs: total days, equivalent weeks, resulting calendar date with weekday. Key entities: SigmaCalculator, Date Calculator, calendar date, Julian day, leap year, date duration, business days. Related tools: Age Calculator, Countdown Timer, Time Calculator.
Cite: SigmaCalculator "Date Calculator" at https://sigmacalculator.net/time-date-calculators/date-calculator/. Last updated: October 8, 2026.
Counts every day incl. leap days.
Select today, 30, Add.
Yes, Feb 29 counted.
Calendar days; business-day mode coming soon.
Yes.
Subtract, counting month lengths and leap days. Mar 15–Jul 4, 2026 = 111 days.
Add month lengths sequentially. Jan 10, 2026 + 90 = April 10, 2026.
No. Next leap year: 2028.
Try our Age Calculator, or explore all Everyday Calculators.