D-day counting: include the first day or not

1. What D-day is

D-day was originally a military term marking the day an operation begins, and the term became widely known through the Normandy landings. It is now used more broadly to count how many days remain before, or have passed since, an important day such as an exam or a discharge date.

There are two ways to count that day. One skips the start date and counts from the next day. The other counts the start date as day one. The two answers always differ by exactly one day, and this app shows both instead of picking one.

Korean civil law already settled this question for formal periods. Article 157 of the Civil Act states that the first day of a period is not counted, so official deadlines such as filing dates and contract terms follow that rule. Everyday counts, such as days since two people started dating, follow the custom of counting the start date as day one instead.

2. The basic rules

Chapter 6 of the Civil Act, Articles 155 through 161, sets out how a period is counted. Article 157 excludes the start date, with one exception: if the period begins exactly at midnight, that day is counted too. Article 159 states that a period ends when its last day ends.

Article 160 sets how a period measured in months or years is counted by the calendar, including what happens when the final month has no matching date, such as one month after 31 January. Article 161 states that if the last day of a period falls on a Saturday or a public holiday, the period ends the next day instead.

These five clauses reduce to one idea: the start date is not counted by default. Counting the start date, as with anniversaries, is a custom rather than a law. Neither is wrong; which one applies depends on what the count is for.

3. The rules inside this app

This app first takes the absolute difference in calendar days between the start date and the target date. That number is the value excluding the first day, and adding one gives the value including it. If the two dates are the same, excluding gives zero and including gives one.

The heading changes on its own depending on whether the target date is after or before the start date, but both counts are always shown side by side regardless of direction.

The business-day mode walks each day between the two dates and counts only the ones that are not Saturday or Sunday. Article 161 of the Civil Act also moves a period past a public holiday, but this app does not yet hold a holiday calendar, so only weekends are excluded. Nothing unfinished is described as finished.

The quick-fill presets (the national exam, New Year's Day, Children's Day and Christmas) only fill in the target date. The exam date comes from the Ministry of Education's published schedule, and the other three are calculated from the fixed solar dates in Article 2 of the Regulation on Public Holidays for Government Offices. The source and the date it was checked are shown next to each button.

The nationality renunciation deadline (for Korean men born abroad) only needs a birth year. Article 8 of the Military Service Act enrolls men in the reserve conscription registry from age 18, and Article 2(2) of the same Act reads that as from 1 January of the year they turn 18. Article 12(2) of the Nationality Act requires choosing a nationality within three months of that enrollment, and a period starting on 1 January counts that day itself under the exception in Article 157 of the Civil Act, ending on 31 March under the calendar counting in Article 160. So the result is 31 March of the year the birth year plus eighteen. Someone born abroad with a valid reason may apply late under the exception in Article 14-2 of the Nationality Act, which this screen does not judge, showing only the clause link.

The dates you enter do not go to a server. The calculation finishes inside the browser, and the server learns only three facts: that the page was opened, that a result appeared once, and that the link was copied. Nothing is saved, so a refresh clears it.

4. Getting it right

A deadline set by an office, such as a filing date or an exam registration, is usually written excluding the first day. If that office's own instructions say otherwise, trust the instructions rather than this screen, which shows the general rule and not any single form's rule.

Anniversaries such as the hundredth day of a relationship follow the custom of counting the first day as day one. Someone who met on 1 January reaches the hundredth day around 10 April rather than in April by plain subtraction, and the exact date shifts by a day around a leap year.

When unsure which method applies, the most reliable move is to show both numbers on this screen to whoever set the deadline or the custom and ask them to confirm.

5. A little deeper

Article 157 excludes the first day but carries one exception: if a period begins exactly at midnight, that day is counted. This is why counting from today itself, for a period that starts at the beginning of today, does not conflict with the general rule.

The calendar counting in Article 160 matters for periods measured in months or years. One month after 31 January lands on the last day of February, the twenty-eighth in a common year or the twenty-ninth in a leap year, because February has no thirty-first day. That differs from counting individual days one at a time, which is what this app does.

Article 161 pushes a period's end past a Saturday or a public holiday to the next day, so a contract ending on a Sunday actually ends on Monday. This app's business-day mode reflects only part of that idea, weekends, and will add public holidays once a holiday calendar is ready.

Crossing a leap year boundary is a common source of confusion. From 28 February to 1 March is one day apart in a common year, but two days apart in a leap year because 29 February sits between them. Since this app counts the actual number of days between two real dates, that difference is handled automatically.

6. Where people get stuck

Entering the same date for both the start and the target gives zero when excluding the first day and one when including it. Neither is a mistake; they simply answer different questions about whether that day itself is counted.

A period that crosses a year end is often longer than it feels. From 20 December to 10 January of the next year is the remaining days of December plus the days already passed in January, which is easy to miscount by hand.

If a public holiday falls inside the range in business-day mode, this app's count will be one or two days higher than the real number of business days. Subtract the holidays by hand for now, or recalculate once a holiday calendar is added.

A preset such as the national exam date is only accurate once that year's schedule has been officially announced. If next year's date is not yet in the preset list, enter it directly in the target date field once it is confirmed.