Calendar Calculator
Choose between two modes: find the number of days, weeks, months and years between two dates, or add (or subtract) a set number of years, months, weeks and days to any date. Toggle business days only to skip weekends. Results update instantly as you type.
How to calculate days between two dates
The simplest way is to convert both dates to the same epoch timestamp and divide the difference by 86,400 (the number of seconds in a day). The formula in plain terms is: total calendar days = (end date - start date) in days. This calculator does that instantly for any two dates you enter. You can also toggle "include end date" to add one more day, which matters when you are counting an inclusive range (for example, a five-day trip that starts on Monday and ends on Friday inclusive is 5 days, not 4). The result also breaks down into years, months, weeks and remaining days, so you can express the gap in whichever unit is most useful.
Business days and working-day calculations
A calendar-day count includes weekends, but many practical tasks (project deadlines, legal notices, payment terms, shipping estimates) are expressed in business days. Business days are simply the weekdays, Monday through Friday, within the date range. This calculator counts them automatically in the date-difference mode, alongside the total calendar days. In add/subtract mode, the "Business days only" toggle makes the days and weeks fields count weekdays only, skipping over Saturdays and Sundays when stepping forward or back. Note that public holidays vary by country and are not excluded here; you would need to subtract them manually for precise working-day counts in your jurisdiction.
Adding and subtracting periods from a date
Switch to "Add / subtract from a date" to find the date that is a specified number of years, months, weeks and days before or after a starting date. Year and month arithmetic follows calendar rules rather than fixed day counts: adding one month to January 31 gives February 28 (or 29 in a leap year) rather than March 2 or 3, because months have different lengths. Adding years handles leap years the same way: one year after February 29 is February 28 of the following year (or February 29 if the next year is also a leap year). This makes the result correct for contractual and statutory time limits, where "one month" means the same calendar date in the next month.
Leap years and calendar edge cases
A leap year has 366 days instead of 365, with February 29 added. Leap years occur in years divisible by 4, except that century years (divisible by 100) are not leap years, unless also divisible by 400. So 2000 and 2400 are leap years; 1900 and 2100 are not. When you calculate a date span that crosses February in a leap year, those extra 24 hours are counted automatically. The ISO week-number system (ISO 8601) means the first week of a year is the week containing the first Thursday, so week 1 can start in late December and the year can have 52 or 53 weeks.
Common date spans at a glance
| Interval | Calendar days | Approximate business days | Approximate weeks |
|---|---|---|---|
| 1 week | 7 | 5 | 1 |
| 2 weeks | 14 | 10 | 2 |
| 1 month (avg) | 30 | 22 | 4.3 |
| 1 quarter (3 months) | 91 | 65 | 13 |
| 6 months | 181 | 130 | 25.9 |
| 1 year | 365 | 261 | 52.1 |
| 1 leap year | 366 | 262 | 52.3 |
| 2 years | 730 | 522 | 104.3 |
| 5 years | 1826 | 1305 | 260.9 |
Exact counts for frequently used intervals, assuming a standard (non-leap) year.
Frequently asked questions
How do I count the number of days between two dates?
Enter the earlier date in the "Start date" field and the later date in the "End date" field. The calculator shows the total calendar days, weeks, business days, and the year-month-week-day breakdown instantly. Toggle "Include end date" if you want to count the final day itself (for example, a project that runs from June 1 to June 30 is 29 days not including June 30, or 30 days including it).
How do I find a date 90 days from today?
Switch the mode to "Add / subtract from a date", set the start date to today, enter 90 in the Days field and leave the direction as "Add". The result date updates immediately. If 90 business days is what you need (common for legal deadlines), toggle on "Business days only".
What are business days and why do they matter?
Business days (also called working days) are weekdays: Monday, Tuesday, Wednesday, Thursday and Friday. They exclude Saturdays and Sundays. Many contracts, invoices and legal deadlines (such as "payment due within 30 business days") are measured in business days rather than calendar days. This calculator shows both for date-difference mode, so you can use whichever applies.
Why does adding one month to January 31 give February 28, not March 2?
Calendar month arithmetic works by moving the month number forward and keeping the day number, but clamping it to the last valid day if it would overflow. January 31 + 1 month moves to February, which has only 28 days (or 29 in a leap year), so the result is February 28 (or 29). This is the standard behavior used in contracts and date-based rules, where "one month" means the corresponding date in the next month.
Does the calculator account for leap years?
Yes. All calculations use the Gregorian calendar, which includes leap years (every 4 years, except century years not divisible by 400). February 29 is counted as a real calendar day when it falls within your date range, so a span crossing a leap-year February correctly shows 366 days for the year.
What is an ISO week number?
The ISO 8601 standard defines the first week of the year (week 1) as the week containing the year's first Thursday. This means week 1 can start in late December, and the last week of the year can end in early January. The result date in "add/subtract" mode shows the ISO week number, which is useful for scheduling in industries like retail, manufacturing and logistics that plan by week numbers.