D-Day Calculator

About the D-day calculator

D-day is the big day: an exam, a trip, a wedding, a launch. People count down to it as D-100, D-30, D-1, then D-Day, and after it D+1, D+2. In Korea this is part of daily life, and a 디데이 계산기 (D-day calculator) is something people use every day.

This tool works in two ways:

• Count down to a day — see D-23, D-Day or D+5, and the dates of D-100, D-50, D-30 and more before it
• Count from a start day — see which day you are on today (day 1,045), and the dates of 100 days, 200 days … 1000 days and 1 to 10 years. For couples, a new baby, a new job or a habit

Features

• D-day label — D-23, D-Day or D+5, big enough to screenshot
• Countdown marks — the dates of D-365, D-100, D-50, D-30, D-14, D-7, D-3 and D-1 before your day
• Milestone list — 100 to 1000 days and 1 to 10 years, with the date, the weekday and the days left
• Day 1 or day 0 — count the start day as day 1 (the Korean way) or as day 0
• More milestones — up to 10,000 days and 50 years
• Copy the list — paste all dates into a chat, a note or a calendar
• Saved for next time — your days stay in this browser

How D-day counting works

The number after D- is the number of days left. D-1 is the day before, D-Day is the day itself, and D+1 is the day after.

So if your exam is on 1 January 2027 and today is 11 November 2026, it is D-51. D-100 was on 23 September 2026.

Day 1 or day 0?

When you count from a day, there are two ways:

• The Korean way: the start day is day 1. The day you started dating, or the day a baby was born, is day 1. So the 100th day (100일) is 99 days later. A baby's 백일 (100th-day party) is counted this way too
• Day 0: the start day is day 0, and day 100 is 100 days later. This is how many other date tools count

Years are the same in both ways. The 1st anniversary (1주년) is the same date one year later: 1 January 2027. In the day 1 way that is day 366, because 2026 has 365 days.

Popular milestones

• Couples — 100 days (100일), 200 days, 300 days, 1 year (1주년), 500 days, 1000 days
• Babies — 100 days (백일), then the first birthday (돌) at 1 year
• Exams — D-100, D-50, D-30 and the last week
• Habits — 30 days, 100 days and 1 year without smoking, of running or of learning a language
• Work — 100 days in a new job, 1000 days of a project

How to use it

• Choose Count down to a day or Count from a start day
• To count down, pick the target day, and give it a name if you like
• To count up, pick the start day, and choose day 1 or day 0
• Read the big number: D-23, D-Day, D+5, or Day 1,045
• Look at the list for the dates of D-100, 100 days, 1 year and more
• Press Copy the list to share it

Frequently asked questions

What does D-day mean?

D-day is the day something big happens. D-10 means 10 days before it, and D+10 means 10 days after it. The word comes from the military, where the "D" simply stands for "day".

How do I count 100 days from the day we met?

Choose Count from a start day, pick the day you met, and keep day 1 (Korean way). The 100 days row shows your 100th day. It is 99 days after the start day, because the first day counts as day 1.

Why is my 100th day one day earlier than in another app?

That app counts the start day as day 0. Switch to Start day = day 0 here and the dates will match it.

Is the 1-year anniversary day 365?

Not quite. The anniversary is the same date one year later. In the day 1 way that is day 366 in a normal year, and day 367 when the year has a 29 February in it. This tool shows the day number next to each anniversary.

What happens with a start day on 29 February?

In a year without 29 February, the anniversary moves to 28 February.

Does it work for a date in the past?

Yes. For a countdown it shows D+ and how long ago it was. For a start day it shows the day you are on and which milestones have already passed.

Good to know

• The day count follows the date on your device, and moves on by itself at midnight
• Time zones do not change the dates: the count is by calendar days
• Your dates and names are saved only in this browser
• Nothing you type is sent anywhere

Related tools

• Countdown to Date — a live clock down to the second
• Date Calculator — add days to a date, or count the days between two dates
• Korean Age Calculator — 만 나이, 세는 나이 and 연 나이
• Week Number — what week it is today
• ICS File Maker — put your D-day into your calendar with a reminder
• Lunar Calendar Converter — lunar birthdays and holidays