Two different rules
- 100th day — the birth day is day 1, so the 100th day is birth date + 99 days. Korean families mark it as baek-il with a small celebration.
- “100 days old” — some people mean birth date + 100 days instead. Decide which one you mean before booking anything.
- First birthday — the same date one year later, not day 365.
Date table by birth date
For a baby born on March 10, 2026:
| Milestone | Date | How |
|---|---|---|
| 50th day | Apr 28, 2026 | birth + 49 days |
| 100th day | Jun 17, 2026 | birth + 99 days |
| 200th day | Sep 25, 2026 | birth + 199 days |
| First birthday | Mar 10, 2027 | same date next year |
| 1,000th day | Dec 3, 2028 | birth + 999 days |
Counting 99 days on a paper calendar is error-prone; a spreadsheet cell with =birth_date+99 is safer. See days-until formulas.
Count down to the preparations too
The party date is rarely the tight one. Venue booking, a photo session and ordering favors all have their own deadlines. Add each deadline as its own countdown. If you hold the party on a nearby weekend, add both the actual milestone and the party date.
Adding them to the app
D-Day Counter shows target date − today in calendar days. The target day itself shows D-DAY, the day before shows D-1, and the day after shows D+1.
Enter the calculated 100th-day date
In D-Day Counter, tap “Add New D-Day”, title it “100th day” and set the Target Date to birth + 99 days. If you enter the birth date instead, the 100th day will show as D+99.
Birthdays repeat yearly
Enter the birth date, choose the Birthday category and set Repeat to Yearly. The app always shows the days left until the next birthday.
See what is coming up
The Stats tab lists the closest upcoming dates in “Upcoming TOP 5”.
The app’s English screenshots below still show sample titles in Korean (an exam and a wedding anniversary).
FAQ
Do I count from the due date for a premature baby?
Family celebrations usually use the actual birth date. For anything medical, such as developmental checks, follow your care team’s guidance.
What about a half birthday?
It is a date, not a day count: the same day six months after birth.
D-Day Counter · Days Until / Since
No sign-up: just a title and a date. Everything stays on your phone. Free on Android, in English and Korean.