Milestone date calculator
What day you are on, and when day 100 and day 1000 fall.
Korean counting of the hundredth and thousandth day treats the first day as day one. Counting from 1 January, day 100 is 10 April, not the 11th. Birthdays and age work the other way, placing the day of birth at zero — two conventions meeting in the same place, which is where the off-by-one comes from. This screen lets you pick which one and says which it used.
Start date
The day you met, were born, or began — anything.
The Korean convention for day 100 and day 1000 — day 100 is start + 99.
Day count
Age places birth at zero while day counting starts at one, so a one-day difference between them is expected.
Calendar arithmetic. Lunar-calendar anniversaries cannot be worked out here.
From 1 January, day 100 is 10 April
With the first day counted as day one, day N is the start date plus N−1. Enter 1 January 2026 and day 100 falls on 10 April, day 200 on 19 July, day 500 on 15 May 2027 and day 1,000 on 26 September 2028. Switching to elapsed days pushes every row back by one day, so day 100 becomes 11 April. The gap between the two conventions is exactly one day on every row, and the only row the table highlights is the one that lands on today.
Day 365 and the first anniversary are different days
The 365 row is a count of days; an anniversary is the same calendar date. Counting from 1 January 2026, day 365 is 31 December, and the anniversary on 1 January 2027 is day 366. A 29 February in between widens the gap by another day — from 1 June 2027, 1 June 2028 is day 367. If you think of the 365-day mark and the anniversary as the same day, this is where the extra day comes from.
Years, months and days do not depend on the counting mode
That card counts the way age is counted, with the start date at zero. When today's day of the month is smaller than the start's, a month is borrowed, and the number of days borrowed is the length of the month before today. From 15 August 2025 to 4 September 2026 is 1 year, 0 months, 20 days: 15 cannot be taken from 4, so a month is given back and August's 31 days are added. Choosing inclusive counting does not change this figure; it only changes the day count and the table.
What this screen does not do
It drops the time of day. Both the start and today are set to midnight before subtracting, so the answer is the same whatever hour you began. Today is this browser's date, so opening the page in another time zone can shift it by a day. Lunar-calendar anniversaries cannot be worked out, and the table ends at day 10,000 (about 27 years and 4 months). Milestones already reached stay in the table, so you can look back at when they fell.
A date that has not arrived yet gives a negative day count
The table still stands when the start date is after today. If today is 4 September 2026 and you enter 14 September, the count reads day −9 with the first day as day one, or day −10 in elapsed mode, and every row of the table shows days remaining — day 100 falls on 22 December, 109 days away. The years-months-days card, however, subtracts the start from today, so for a date still in the future it means nothing. Clear the date field and the count runs from 1 January 2026.
The next milestone is the first row not yet passed
The table has nine rows — 100, 200, 300, 365, 500, 1,000, 2,000, 3,000 and 10,000 — and the first of them with zero or more days remaining goes into the next-milestone card. With today at 4 September 2026, entering 1 January 1998 gives day 10,474: all nine rows have passed and the card disappears.
Common questions
QWhy is day 100 only 99 days later?
Because the first day counts as one. The start date is already day one, so day 100 is the start plus 99. Choosing elapsed days instead treats the start as zero and adds a full 100 — the western anniversary convention.
QWhy does this differ from age?
Age places birth at zero and adds a year at each birthday; day counting starts at one. A one-day difference between the two is expected.
QWhy is there no day 400 or second anniversary in the table?
The table lists only the milestones commonly marked in Korea, so after 365 comes 500 and there is no 730. A second anniversary is not a day count at all — it is the day the years-months-days card reads 2 years, 0 months, 0 days — so it belongs to no row. For dates like those, work out what falls a given number of days after the start in the date calculator instead.