Date Difference Calculator
Find the exact interval between two dates in years, months and days, with total days and business days. Or start from a date and add or subtract a period.
Educational use: results are estimates made for study and reference. Checking the answer and deciding what to do with it is the sole responsibility of whoever uses the tool. Read the legal notice.
On this page
- How to use the calculator
- Worked examples
- How the calculation works
- Useful calendar numbers
- One interval, three ways to count
- Before you rely on a deadline
- Pitfalls in counting dates
- Common questions about counting dates
- What it is for
- Where our calendar comes from
- Calendar curiosities
- Calendar terms
- More everyday tools
How to use the calculator
There are two modes, chosen in the first field.
Difference between two dates
- Enter the start date (by default, it is filled in with today's date).
- Enter the end date. If it is earlier than the start date, the tool swaps the order and lets you know.
- Click Calculate. The result comes in years, months and days, with the total number of days, the total number of weeks, business days and the total number of hours.
Add or subtract a period
- Enter the starting date.
- Choose add or subtract, type the amount and choose the unit (days, weeks, months or years).
- The tool shows the resulting date, along with the day of the week.
If your browser does not show a calendar picker, type the date in the format yyyy-mm-dd or dd/mm/yyyy.
Worked examples
1. Between any two dates
From March 15, 2024 to September 20, 2025.
Of those 554 days, 396 are business days (Monday to Friday), without subtracting holidays.
2. How long until Christmas
On January 1, 2024, how many days were left until December 25, 2024?
3. Ninety days later
A 90-day warranty, counted from January 1, 2024, ends when?
Notice the effect of the leap year: without February 29, the date would fall on April 1.
4. The 31st trap
What is 1 month after January 31, 2024?
This rule, staying on the last available day of the target month, is the most common in practice, but different contracts and systems may use other conventions.
5. Six weeks of classes
A course starts on October 20, 2024 and lasts 6 weeks.
How the calculation works
Counting days is simple. Counting months and years is the hard part, because they have different lengths. The calculator does it like this:
Years, months and days
- It calculates how many complete months fit between the dates, counting from the start date.
- It adds those months to the start date and checks how many days are left until the end date.
- The complete months are turned into years (every 12) and remaining months.
Business days
We count Monday to Friday, including the start date and excluding the end date, just like counting the number of nights in a hotel. Holidays are not subtracted, since they vary by country, state and city.
Leap year rule
except those divisible by 100,
unless they are divisible by 4002024 and 2000 are leap years; 1900 and 2100 are not
Useful calendar numbers
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in a leap year) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
To remember: the months with 30 days are April, June, September and November. All the others have 31, except February.
| Period | In days |
|---|---|
| 1 week | 7 |
| Half-month (quinzena) | 15 (or 14, depending on usage) |
| 30, 60 and 90 days | about 1, 2 and 3 months |
| 6 months | 181 to 184 days |
| 1 year | 365 (366 if a leap year) |
| 4-year cycle | 1,461 |
One interval, three ways to count
The same interval can be expressed in different ways. The calculator shows all of them, because each situation calls for a different one.
Before you rely on a deadline
The tool does correct calendar math, but it should not be used to set official legal, contractual, court, tax or school deadlines. Those deadlines have their own rules about which day counts, whether they are calendar days or business days, which holidays are excluded and what happens when the last day falls on a weekend. Always check the applicable rule and, when there are consequences, consult a professional or the responsible authority.
Business days only consider Monday to Friday. Calculations use full dates (without time of day) and apply to the Gregorian calendar, in the range from 1000 to 9999.
Bottom line: this tool is educational. Results do not replace your own checking, and how you use them is entirely your responsibility. More details in the legal notice.
Pitfalls in counting dates
Counting the start date or the end date twice
From the 1st to the 10th is a difference of 9 days, but 10 days if you count both ends. In contracts and deadlines, always check which rule applies: whether or not the starting day is counted.
Assuming every month has 30 days
This causes an error of up to 5 days per half-year. A "6 months" deadline starting on August 31 does not always end on February 28; by the last-day-of-the-month rule, it ends on February 28 or 29, depending on the year.
Forgetting the leap year
Between February 28 and March 1, there is 1 day in common years and 2 in leap years. This can shift a 365-day deadline by one day.
Ignoring time zones and time of day
This tool works only with dates (day, month and year). If your case involves times of day, time zones or daylight saving time, the calculation needs more care.
Thinking business days include holidays
The business day count here only subtracts Saturdays and Sundays. National, state and local holidays, as well as optional days off, vary and are not taken into account.
Common questions about counting dates
How do I calculate how many days are between two dates?
Subtract the start date from the end date. The tool does this and also shows the result in weeks, months and years. Between January 1, 2024 and December 25, 2024, there are 359 days.
Are the start and end dates included in the count?
Here, the total number of days is the difference between the dates (in practice, the start date is not counted and the end date is). If you need to count both ends, add 1 to the result.
How do I calculate business days?
By subtracting Saturdays and Sundays from the interval. The tool counts Monday to Friday, but does not subtract holidays, which vary by region.
How do I add 30, 60 or 90 days to a date?
Use the "Add or subtract a period" mode, choose days and type the amount. The tool already accounts for months of different lengths and leap years.
What happens if I add 1 month to January 31?
Since February has no 31st, the date lands on the last day of the month, that is, February 28 (or 29, in a leap year).
How many weeks are in a year?
52 weeks and 1 day (or 2 days in a leap year), since 365 ÷ 7 = 52.14.
How can I tell if a year is a leap year?
If it is divisible by 4 but not by 100, unless it is also divisible by 400. 2024 is a leap year, 2100 is not and 2000 was.
Can I calculate my exact age here?
Yes. Use your date of birth as the start date and today as the end date. The result comes in years, months and days.
Why does "1 month" not always have the same number of days?
Because months have from 28 to 31 days. In the calculator, a month is a calendar month, not a fixed 30 days.
Is the calculator suitable for court or contract deadlines?
No. Legal and contractual deadlines have their own rules, and this tool is for educational use only. Always confirm with a professional or the responsible authority.
What it is for
- Countdowns: how many days are left until a trip, an event or an exam.
- Study and project planning: spreading tasks out until a final date.
- Informal deadlines: store warranties, trial periods, returning loans.
- Anniversaries: how long a relationship, a marriage, a company or a friendship has lasted.
- Age in days or months: how old someone or something is, in small units.
- Workouts and goals: 8-, 12- or 16-week plans.
Where our calendar comes from
The calendar we use is the Gregorian calendar, introduced by Pope Gregory XIII in 1582 to correct a drift in the Julian calendar: because the Julian year was slightly longer than the solar year, dates had moved away from the seasons. That year, ten days were "erased" (October 4 was followed by October 15).
The leap year rule we use today, with the exception for century years not divisible by 400, came out of that very reform. Catholic countries adopted the new calendar quickly; England and its colonies only did so in 1752, when eleven days had to be skipped.
Calendar curiosities
- Anyone born on February 29 only has a "real" birthday every 4 years, and the odds of that date are about 1 in 1,461.
- The day of the week repeats every 28 years in the Gregorian calendar (within the same century), because of how common and leap years fit together.
- September comes from "septem" (seven), but it is the ninth month. October, November and December have the same "mismatch": the ancient Roman calendar began in March.
- There are 52 weeks and 1 day (or 2, in a leap year) in a year, which is why the day of the week of a given date moves forward each year.
Calendar terms
- Leap year
- A year with 366 days, in which February has 29 days.
- Calendar days
- A count that includes every day on the calendar, including weekends and holidays.
- Business days
- Working days, usually Monday to Friday, excluding holidays.
- Gregorian calendar
- The calendar used in most of the world, introduced in 1582.
- Interval
- The period between two dates.
- Quinzena
- A period of about fifteen days (a half-month).
- UTC
- Coordinated Universal Time, a reference used to avoid differences caused by time zones and daylight saving time.