Date Calculator — Days Between & D-Day
Count days between dates, add or subtract days, D-days and 100-day anniversaries, weekdays excluding Korean public holidays (2026–2027) and Korean school graduation years.
How many days between two dates?
The D-day shows the plain difference; the include setting only affects the day count.
Enter dates and the values to calculate.
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Date Calculator — Days Between & D-Day: complete guide
Walk through the steps and examples, or jump straight to the topic you need.
At a glance
- What it calculates
- The difference between two dates and D-Day, the date a number of days before or after, lists of 100-day and 1st anniversary dates, weekday working days, and school entry and graduation years from a birth date.
- Date basis
- Calculates in whole dates rather than time zones or times of day. When working days are subtracted, weekends and holidays still each count as a day in the calendar.
- Anniversaries
- The day you start counts as day 1. Day 100 is 99 days after the start date.
- Working days
- Enter both end dates to count Monday through Friday. For 2026 and 2027 it subtracts Korean holidays including Seollal, Chuseok, and substitute holidays, and for other years only Korean public holidays with fixed dates.
- Include option
- Choose whether both end dates are included in the number of days in a period. It does not apply to D-Day.
- Date shifting
- Supports whole numbers of days between -365,000 and 365,000.
- Result range
- Dates from year 0001 to 9999, and you can check and copy the result and the basis of the calculation.
How to use Date Calculator — Days Between & D-Day
Choose between difference and date shifting
Use date difference when you want to know the period between two dates. Choose date shifting to find a single date, such as 100 days from today. D-Day is interpreted together as the difference between the base date and the target date.
Enter the dates to use as the basis
For a period, enter two dates; for date shifting, enter a base date and the number of days to shift. In date shifting, a negative number means an earlier date and a positive number a later date. It counts day by day rather than shifting by months.
Decide whether to count both end dates
Turn on the include option when the first and last days should both count as one day. Turn it off to get only the gap between dates. The gap within the same day is 0 days, but counted as a one-day schedule it becomes 1 day, which is the difference.
Check the dates and basis in the result
The number of days in a period and D-Day can look like different numbers because their bases differ. If a separate rule applies, such as an application deadline or booking condition, check the weekend, holiday, and cutoff time details in that notice before relying on the result.
What it’s good for
Date difference and whether to count both end dates
When calculating a period, first decide whether the start and end dates are included. For example, the difference from January 1 to January 3 is 2 days, but counting both end dates gives 3 days. In this tool, you can choose whether to include both end dates in the date difference result.
D-Day and date shifting
D-Day is shown as the difference between two dates, and once the target date has passed it appears as D+. For date shifting, a positive number gives a date after the base date and a negative number gives a date before it. It uses a calendar basis, where weekends and holidays each count as a day.
Counting 100 days and 1st anniversaries with the first day as day 1
For dating anniversaries or a baby's 100th day, the day you start counts as day 1. So if you started on January 1, day 100 is April 10. When you pick anniversaries, it shows the dates from 100 days up to 3,000 days and the 1st, 2nd, 3rd, 5th, and 10th anniversaries, along with the D-day from today, all at once.
Working days and school years in Korea
For working days, enter both a start and an end date to count Monday through Friday; when switched on, it also subtracts Korean public holidays and substitute holidays that fall on weekdays. For 2026 and 2027 it covers Seollal (Lunar New Year) and Chuseok too, and for other years it subtracts only the public holidays with fixed dates. For school years, following Article 13 of Korea's Elementary and Secondary Education Act, it shows the Korean school entry and graduation years for elementary, middle, high school, and university, based on entering in March of the year after the child turns 6.
Worked examples
January 1 to January 3
Start 2026-01-01
End 2026-01-03Date difference 2 days
Both ends included 3 days · D-2Shifting a date across February in a leap year
Base 2024-02-28
Days to shift 22024-03-01Difference between period, D-Day, and date shifting
Swipe the table sideways to see the rest.
| Feature | Input | What the result means |
|---|---|---|
| Date difference | Two dates and whether to include both ends | The gap between dates, or the period counting both ends |
| D-Day | A base date and a target date | D- if the target is in the future, D+ if it has passed, D-day if it is the same day |
| Date shifting | A base date and a whole number of days | The shifted date, counting weekends and holidays |
| Anniversaries | The day you started | Dates for 100 to 3,000 days and the 1st, 2nd, 3rd, 5th, and 10th anniversaries, with D-day |
| Working days | Two dates and whether to subtract holidays | The number of days from Monday through Friday, minus holidays |
| School years | Birth date | Entry and graduation years for elementary, middle, high school, and university in Korea |
When the result isn’t what you expected
The calculation is off by one day
Check the include-both-end-dates option first. A gap that treats the start date as day 0 and a schedule that counts it as day 1 differ by one day. D-Day is calculated as a date difference regardless of the include option.
Weekends are counted too
Date difference and date shifting count every day on a calendar basis. To count only weekdays, choose working days and turn on subtract public holidays. Company days off and temporary public holidays must be subtracted separately.
A date or shift value gives an error
Check whether the date actually exists, which February 30 does not. The number of days to shift can't be a decimal and has a range limit. If the shifted result falls outside the years 0001 to 9999, adjust the base date or the number of days.
Key terms
- Date difference
- The number of days that must pass to get from one date to another.
- Both ends included
- A way of counting where the start date and the end date each count as a day in the schedule.
- Calendar days
- A basis that counts every date on the calendar, whether it is a weekend or a holiday.
Date Calculator — Days Between & D-Day FAQ
Can I exclude business days or holidays?
Choosing working days leaves out weekends, and turning on subtract public holidays also removes Korean public holidays and substitute holidays that fall on weekdays. For 2026 and 2027 it removes them as officially announced in Korea, including Seollal, Chuseok, and Buddha's Birthday, and for other years it removes only public holidays with fixed dates. Please check company days off and temporary public holidays separately.
Does D-Day change if I change the include-both-end-dates option?
D-Day is calculated as a date difference regardless of the include option. The include option applies when counting the days in a period.
How far can date shifting go?
You can shift from the base date by whole numbers of days, from -365,000 to 365,000. The result must fall between the years 0001 and 9999.
Does D-Day change automatically after today passes?
The result is calculated from the base date you entered on the screen. If a page has been left open for a long time, check that the base date is the one you want and correct it.
Does it calculate differences in hours or minutes too?
It calculates in whole dates only. It does not provide hour or minute differences between two times, or time zone conversions for a specific region.