Date Difference Calculator
Number of days between two dates.
Days between the two dates
0days
Enter a start and end date.
What is a date difference calculator?
This tool calculates the number of days, weeks, and months between two dates — useful for trip planning, contract periods, D-day countdowns, and anniversaries. You can toggle whether the start and end dates themselves are counted.
How it is calculated
Days = (end date − start date) ÷ 24h; add 1 day if both endpoints are included.
Weeks = days ÷ 7, Months (approx.) = days ÷ 30.44.
Inclusive example
| Mode | Result |
|---|---|
| Exclusive (default) | 4 days |
| Inclusive | 5 days |
"Exclusive" vs "inclusive" counting — why D-day can be off by one day
There are two common ways to count the gap between dates. Exclusive counting measures only the elapsed time between the start and end points, while inclusive counting treats the start date itself as day one. The same two dates can produce results one day apart depending on which convention is used.
D-day notation is a familiar example: calling the event date itself "D-day" and the day before "D-1" follows inclusive-style counting. In this calculator, a start date of 2026-01-01 and an end date of 2026-01-05 with the inclusive toggle off (the default) gives 4 days — the pure elapsed time (24h × 4) between the two points, not "which day number is January 5." Counting inclusively instead — "January 1 is day 1, so what day number is January 5?" — gives day 5, matching the result when the inclusive toggle is switched on.
Which convention to use for tenure days or days of medication
Real-world conventions vary by context. Tenure (days employed) is usually counted inclusively, treating the start date as day one: starting on 2026-01-01 and working through 2026-01-05 is commonly counted as 5 days, not 4. Days of medication follows the same logic — the first day a dose is taken usually counts as day one.
By contrast, interest-accrual periods or the exact number of days a lease was used typically call for exclusive counting, where only one endpoint is included. Toggling this calculator's inclusive checkbox on or off lets you match either convention precisely.
How you count a period changes the answer
The gap between two dates looks simple, but whether the first day counts shifts the result by one. Where a single day matters — contract terms, statutory deadlines — that shift changes the outcome.
Excluding the first day
Under the Civil Act, a period set in days, weeks, months or years starts counting from the following day. A seven-day term agreed on 1 March runs from 2 March and ends on 8 March.
There are exceptions: a period beginning at midnight includes the first day, and age counting includes the date of birth.
If the final day falls on a Saturday or public holiday, the period ends on the next working day — a common trap for filing deadlines.
Periods set in months or years
Periods in months or years follow the calendar rather than a day count. One month from 31 January ends on 28 February, or 29 in a leap year; where no matching date exists, the last day of the month applies.
This regularly matters in leases and employment contracts set in months: the number of actual days varies, but expiry is fixed by the calendar.
Business days and calendar days
Calendar days count weekends and holidays; business days exclude Saturdays, Sundays and public holidays. The same 'ten days' can mean two real weeks under the latter.
Administrative processing periods are generally business days, while a contract saying only 'days' is read as calendar days — worth stating explicitly to avoid dispute.
Korean public holidays shift year to year with substitute-holiday designations, so long deadlines need the relevant year's schedule.
Legal basis
- Start of a period — Civil Act art. 157
- Expiry of monthly and yearly periods — Civil Act art. 160
- Final day falling on a holiday — Civil Act art. 161
- Public holidays — Regulations on Public Holidays for Government Offices