Date Calculator – Add, Subtract & Compare Dates

Date Calculator – Add, Subtract and Compare Dates

This free date calculator adds or subtracts any combination of years, months, weeks and days to a date, finds the exact difference between two dates in years, months and days, and gives you full information about any date – its day of the week, ISO week number, day of the year, quarter, whether it is in a leap year and its Unix timestamp. It is built for real-world date maths: it handles leap years, months of different lengths and the awkward "31st of the month" problem correctly, and it lets you choose how month-end dates are treated.

Use it to find a date six months from today, work out a date of birth that makes someone 18, calculate a warranty or contract end date, find the day of the week you were born on, or check which week number a date falls in for payroll and project planning.

How to Use the Date Calculator

1. Pick a mode. Add / subtract moves a date forwards or backwards. Difference measures the time between two dates. Date info tells you everything about one date.
2. Enter the start date or tap Today.
3. To add or subtract, choose + or −, then type years, months, weeks and days – or tap a quick button. You can combine them, for example 1 year, 2 months and 10 days.
4. To compare dates, enter the end date and decide whether to include it.
5. Read the result. The new date is shown with its day of the week, and the panel below gives week number, day of year and other details. Copy the summary with one click.

How Date Addition Works

The calculator applies units from largest to smallest: first years, then months, then weeks and days. This matches how people and most software think about dates – "one year and one day after 28 February 2027" is 29 February 2028, because adding the year first gives 28 February 2028 and then one more day lands on the leap day.

Adding days and weeks is always exact, because every day has the same length in the calendar. Adding months and years is different, because months have 28, 29, 30 or 31 days. Adding one month to 15 March gives 15 April – easy. But what is one month after 31 January? There is no 31 February, so the calculator has to decide:

Calculation	Last day of month (default)	Roll over
31 Jan 2026 + 1 month	28 Feb 2026	3 Mar 2026
31 Mar 2026 + 1 month	30 Apr 2026	1 May 2026
29 Feb 2028 + 1 year	28 Feb 2029	1 Mar 2029
31 Aug 2026 − 6 months	28 Feb 2026	3 Mar 2026

The default "last day of the month" rule is used by banks for monthly payments, by Excel's EDATE function, and by most programming libraries. The roll-over rule is used by some older systems and by JavaScript's built-in Date object, which is why so many simple online calculators give surprising answers for month-end dates.

How the Difference Between Two Dates Is Calculated

In Difference mode the calculator counts whole years first, then whole months, then the remaining days, borrowing from the previous month when needed. From 20 May 2024 to 3 March 2026 is 1 year, 9 months and 11 days. It also shows the same span as total months, total weeks and total days, which is handy for loan terms, notice periods and project plans. For a simple day count with business days and holidays, use our days calculator; to find someone's exact age, use the age calculator.

Week Numbers (ISO 8601)

Many businesses, schools and governments in Europe and Asia refer to weeks by number – "delivery in week 42". Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing 4 January). That means 1 January can belong to week 53 of the previous year, and 31 December can be in week 1 of the next year. A year has 52 or 53 ISO weeks; 2026 has 53, because it starts on a Thursday. The calculator shows the ISO week and ISO week-year for every date.

In the United States, week numbering usually starts on Sunday and week 1 is the week containing 1 January, so US and ISO week numbers can differ by one. Always check which system a schedule uses.

Leap Years Explained

The Earth takes about 365.2422 days to orbit the Sun, so an extra day is added to February almost every four years to keep the calendar in step with the seasons. The Gregorian rules are:

• A year divisible by 4 is a leap year (2024, 2028),
• except years divisible by 100, which are not (1900, 2100),
• except years divisible by 400, which are leap years after all (2000, 2400).

This gives an average year of 365.2425 days – accurate to about one day every 3,200 years. The next leap days are 29 February 2028, 2032 and 2036.

Date Formats Around the World

The same date is written differently in different countries, which causes costly mistakes in contracts, travel bookings and spreadsheets. 03/04/2026 means 3 April in the UK, India and most of the world, but 4 March in the United States.

Format	Example for 27 September 2026	Used in
DD/MM/YYYY	27/09/2026	UK, India, Europe, Australia, most of the world
MM/DD/YYYY	09/27/2026	United States
YYYY-MM-DD (ISO 8601)	2026-09-27	International standard, China, Japan, computing
Long form	Sunday, 27 September 2026	Letters, invitations, legal documents

If you share dates internationally, the ISO format (year-month-day) is the safest, because it cannot be misread and it sorts correctly as text.

Finding the Day of the Week for Any Date

Want to know what day of the week you were born on, or what day Christmas falls on next year? Enter the date in Date info mode. The calculator uses the proleptic Gregorian calendar, which extends today's calendar rules backwards, so results are correct for all modern dates. Mathematicians use formulas such as Zeller's congruence or John Conway's "Doomsday rule" to find weekdays in their heads – the Doomsday rule relies on the fact that 4/4, 6/6, 8/8, 10/10 and 12/12 always fall on the same weekday in any given year. In 2026 that weekday is Saturday.

Unix Time and Day Numbers

Computers often store dates as a Unix timestamp – the number of seconds since midnight UTC on 1 January 1970. The calculator shows the Unix timestamp for midnight UTC at the start of your date, which is useful for developers and database work. It also shows the day of the year (1 to 365 or 366), often used in manufacturing batch codes, scientific data and the "ordinal date" format such as 2026-270.

Date Calculations in Excel and Google Sheets

If you need to repeat a date calculation for many rows, spreadsheets have built-in functions that follow the same rules as this calculator. Dates are stored as serial numbers, so adding days is plain addition, while months and years need special functions.

Task	Formula	Notes
Add 30 days	=A2+30	Works in Excel and Google Sheets
Add 6 months	=EDATE(A2,6)	Uses the last-day-of-month rule
Add 2 years	=EDATE(A2,24)	Years are 12 months each
End of the month	=EOMONTH(A2,0)	Last day of the same month
Difference in years, months, days	=DATEDIF(A2,B2,"Y"), "YM", "MD"	"MD" can be wrong around month ends – check against this calculator
Add 10 working days	=WORKDAY(A2,10,holidays)	Skips Saturday, Sunday and listed holidays
ISO week number	=ISOWEEKNUM(A2)	Monday-start weeks, as shown here
Day of the week	=TEXT(A2,"dddd")	Returns the weekday name

Microsoft itself warns that the DATEDIF "MD" option can return a negative or incorrect number of days when the start date is late in the month. This calculator counts whole months first and then the remaining days, so 31 January to 1 March is always 1 month and 1 day.

Common Uses of a Date Calculator

• Contracts and warranties: a 2-year warranty from the purchase date, or a 12-month lease end date.
• Legal age and eligibility: the date someone turns 18, 21 or 60, or the latest birth date for a school admission year.
• Pregnancy and health: 40 weeks from the first day of the last period, or follow-up appointments in 6 weeks.
• Finance: maturity dates for fixed deposits, loan end dates and payment schedules.
• Projects: sprint and milestone dates, and converting deadlines to week numbers.
• Travel: passport validity rules, such as needing six months left after your return date.

Tips for Getting Date Maths Right

• Say which rule you mean. "One month after 31 January" is ambiguous – write the exact date in contracts.
• Add the bigger units first. Adding 1 month then 1 day can give a different answer from adding 1 day then 1 month near the end of a month.
• Be careful with "within" and "after". "Within 30 days of 1 June" usually means on or before 1 July, not 30 June.
• Watch for leap days. Someone born on 29 February legally turns a year older on 28 February or 1 March depending on the country.
• Use ISO dates when sharing. 2026-09-27 cannot be misread, unlike 09/10/2026.

Frequently Asked Questions