Two ways to count
| Method | Formula | Start day is | Used for |
|---|---|---|---|
| Days since (gap) | today − start | day 0 (D+0) | countdowns, “days until”, “days ago” |
| Day number (inclusive) | today − start + 1 | day 1 | relationship milestones, baby’s 100th day, streaks |
The two always differ by exactly one day. In Korea, couples traditionally celebrate the 100th day counting the first day as day 1, so the 100th day is 99 days after the start and a days-since counter shows D+99 on it.
Milestone date table
Starting on June 1, 2026 (start day = day 1):
| Milestone | Date | Days since start |
|---|---|---|
| 100th day | Sep 8, 2026 | +99 |
| 200th day | Dec 17, 2026 | +199 |
| 300th day | Mar 27, 2027 | +299 |
| First anniversary | Jun 1, 2027 | +365 |
| 500th day | Oct 13, 2027 | +499 |
| 1,000th day | Feb 24, 2029 | +999 |
For any day-number milestone: start date + (N − 1) days.
The first anniversary is not day 365
Anniversaries are counted by calendar date, not by day number. In the example, the first anniversary (June 1, 2027) is the day after day 365 (May 31, 2027), so it is day 366. If a February 29 falls in between, the gap grows by another day.
Setting it up in the app
The Android app D-Day Counter uses the gap method. If you enter the start date, it counts up D+1, D+2 from the next day, so add one to read it as a day number. To see D-DAY on the milestone itself, enter the milestone date.
Enter the 100th day as the target date
Tap “Add New D-Day”, type a title such as “100 days”, pick the date from the table (start + 99 days) as the Target Date and choose the Anniversary category.
Use Yearly repeat for anniversaries
Anniversaries come back on the same date, so enter the start date and set Repeat to Yearly. After each anniversary passes, the countdown moves to the next one.
Pin the one you care about most
The pin button on a list item puts it in the large card at the top of the main screen. One item can be pinned at a time.
The app’s English screenshots below still show sample titles in Korean (an exam and a wedding anniversary).
FAQ
Do some people count the first day as day 0?
Yes. Neither is wrong; agree on one method with whoever you are celebrating with.
Is a baby’s 100th day counted the same way?
Yes, the birth day is day 1. See baby’s 100th day and first birthday.
How do I count how many days or weeks I have lived?
That is a gap count. How many weeks have I lived has spreadsheet formulas and the 10,000-day date.
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.