Ask a young Korean couple how long they have been dating, and you may not hear “three months.” You will hear “오늘 100일이에요”: today is our 100th day.
That sounds simple until you check the calendar. Say a couple started dating on January 1. Most people would add 100 days and land on April 11. But the couple will be celebrating on April 10, a day earlier. Their phone app agrees with them, and so does every Korean friend they ask.
Nobody made a mistake. Korea counts milestone days a little differently, and once you see why, a lot of Korean life makes more sense: the baby parties, the couple rings, and the countdown banners in every high school in August.
Day 1, not day 0
The whole difference comes down to one question: is the starting day day 0 or day 1?
- Counting “days after”: January 1 is day 0. One hundred days later is April 11.
- Counting the Korean way: January 1 is day 1, January 2 is day 2, and so on. The 100th day is April 10, which is 99 days after the start.
It is the same logic as counting the people in a line. The first person is number 1, not number 0. For Koreans, the day a baby is born or a couple says yes is already the first day of the story, so it counts.
It started with babies: baek-il
The Korean love of 100 days began long before dating apps. It began with newborns.
Baek-il (백일, literally “100 days”) is the celebration of a baby’s 100th day of life. For most of history, that was not a small thing. Before modern medicine, infant mortality was high, and a baby who made it through the first 100 days had passed the most dangerous stretch of life. The party was part thanks, part relief, and part prayer for the years ahead.
The traditional 100th-day table carries those wishes in food:
- Baekseolgi (백설기), a snow-white steamed rice cake, for purity and a long life
- Susu-patteok (수수팥떡), sorghum rice cakes coated in red beans, whose red color was believed to keep bad luck away
- Seaweed soup, rice, fruit and a skein of white thread, the thread standing for a life as long as the string
Families also shared the rice cakes with neighbors. The old belief was that the more people ate the baby’s rice cakes, the longer the baby would live.
And yes, the baby’s birthday counts as day 1. A baby born on January 1 celebrates baek-il on April 10. Today the big neighborhood party has mostly shrunk to a family meal and a photo shoot, but the date is still marked on almost every Korean family calendar.
Then couples borrowed it
Somewhere along the way, young couples borrowed the baby’s milestone for themselves. Where an American couple might mark “three months” or “six months,” a Korean couple counts days.
The clock starts on day 1, the day the two officially become a couple (often the day one person confessed and the other said yes). After that, the big dates come in hundreds:
| Milestone | Start: January 1, 2026 | Day of the week |
|---|---|---|
| 100th day (백일) | April 10, 2026 | Friday |
| 200th day | July 19, 2026 | Sunday |
| 300th day | October 27, 2026 | Tuesday |
| 500th day | May 15, 2027 | Saturday |
| 1,000th day | September 26, 2028 | Tuesday |
The 100th day is the first big one. Couples exchange gifts, write letters, and often buy couple rings, matching rings that say “we’re serious” without saying “we’re engaged.” Many couples keep going with the 200th, 300th, 500th and 1,000th days, and the yearly anniversaries are celebrated too.
There is a reason phones matter here. Counting 1,000 days in your head is hard, and forgetting the 100th day is a famous way to get in trouble. That is why so many Korean couples keep a day-counter app on their home screen, showing “우리 함께한 지 237일”: we’ve been together 237 days.
One small warning: the 365th day and the one-year anniversary are not the same day. Counted the Korean way, the 365th day from January 1, 2026 is December 31, 2026, while the one-year anniversary is January 1, 2027. Couples who celebrate both get one extra excuse for cake.
Counting down: the D-day habit
Koreans don’t only count up from a happy day. They also count down to a big one, and they borrowed a military word to do it: D-day.
In a countdown, the big day itself is D-day. The day before is D-1, the day before that is D-2, and the day after is D+1. Here the start is not counted as day 1. D-100 means exactly 100 days to go.
You’ll see D-day numbers everywhere in Korea: on exam posters, wedding plans, job interviews, and the phones of soldiers counting down to their discharge date.
The most famous countdown of all is for the suneung (수능), the national college entrance exam. This year’s exam is on Thursday, November 19, 2026, so D-100 fell on August 11. D-100 is a mini-holiday of its own. Younger students give their seniors snacks and cheering letters, friends trade chocolate and taffy (엿, because 붙다 means both “to stick” and “to pass an exam”), and parents join 100-day prayer meetings at temples and churches.
On October 31, 2026, the banners in Korean high schools read D-19.
Two rules, one calendar
So Korea uses two counting systems side by side, and each one is correct for its own job:
| You want | Rule | Example |
|---|---|---|
| The Nth day of something (baek-il, a couple’s 100th day) | Start date + (N − 1) days | Born October 31, 2026 → baek-il is February 7, 2027 |
| A countdown (an exam, a wedding, a discharge date) | Target date − N days = D-N | Suneung on November 19, 2026 → D-100 was August 11, 2026 |
The first rule counts the first day. The second doesn’t. Mix them up and you will be exactly one day off, which is fine for most things but not for an anniversary dinner.
One more example, because it’s the one people ask about most. A baby born on October 31, 2026 reaches:
- 100 days on Sunday, February 7, 2027
- 1,000 days on Thursday, July 26, 2029
Try it yourself
You don’t have to count on your fingers. Our D-Day & Date Calculator does both kinds of counting:
- The 100-day milestones tab lists the 100th, 200th, 1,000th and other milestone days from any start date. One checkbox switches between the Korean “first day is day 1” method and the “days after” method, so the date always matches the way your family counts.
- The D-day countdown tab tells you how many days are left until any date, in D-day style.
- The days between dates tab answers questions like “how long have we been together?”
Why count in days at all?
Counting in days says something that counting in months doesn’t: every single day counted. A baby’s first 100 days were once a test of survival. A couple’s first 100 days are a test of a different kind. In both cases, Koreans mark the moment by counting from day 1, because the first day was already part of the story.
So if a Korean friend tells you “오늘 우리 100일이야” (today is our 100th day), you’ll know three things: they started counting on day 1, they’re probably holding a gift, and somewhere a phone app has been keeping score the whole time.
Sources
- Korean 100-Day Celebration, USC Digital Folklore Archives: baek-il as a celebration of a baby’s survival when infant mortality was high
- 백일상, 육아크루 육아위키: the foods on the 100th-day table and sharing rice cakes with neighbors
- 백일·돌 D-day 계산기 가이드, 육아크루: the birth day counts as day 1, so baek-il is the birth date plus 99 days
- 5 Fun facts about Korean couple culture, GO! GO! Hanguk: day counting, the 100th to 1,000th days, and couple rings
- Love, Korea and dating, Stars and Stripes Korea: the 100-day anniversary and when couple rings are bought
- D-day 계산기, k-calc: standard D-day notation versus counting the first day as day 1
- 수능 날짜, 2027학년도 수능 2026년 11월 19일, 육아크루: the 2026 exam date, from Ministry of Education and KICE notices
- 수능 D-100… 수험생·학부모 풍속도, 경북매일, August 3, 2009: D-100 gifts, snacks, and parents’ 100-day prayer meetings