How to calculate 100 days of dating

Short answer: if you count the day you got together as Day 1, your 100th day is 99 days after your start date. If you count it as Day 0, it is 100 days after. The two methods are always exactly one day apart.

Day 1 vs Day 0

• Day 1 (inclusive): the first day already counts. This is how most couples count, and it is the standard way in Korea, where the 100-day anniversary is a big celebration.
• Day 0 (elapsed): the first day is the starting line and the count goes up after each full day, like a stopwatch.

Neither is wrong. What matters is that you both use the same one, so you do not plan the dinner a day late.

A worked example

Say you got together on January 1, 2026.

• January has 31 days, so January 31 is Day 31 (Day 1 method).
• February 2026 has 28 days, so February 28 is Day 59.
• March 31 is Day 90.
• Ten more days: April 10, 2026 is Day 100.

With the Day 0 method, the same day is Day 99, and Day 100 is April 11.

Common mistakes

• Adding 100 days to the start date gives the Day 0 answer. If you count the first day, subtract one.
• Assuming every month has 30 days. Months have 28 to 31 days, so "about three months and ten days" can be off.
• Forgetting leap years. February 29 shifts every count after it by a day.

After Day 100

The same rule gives you Day 200, Day 300 and Day 1,000. Many couples also celebrate monthly anniversaries and the yearly one, which are counted by calendar date instead of by days.

Doing this with Duodays

Duodays counts the day you got together as Day 1 by default, and you can switch any counter to Day 0. It lists every 100 days up to Day 10,000 along with monthly and yearly anniversaries, and reminds you a week before, the day before and on the day. No sign-up needed.

Is the first day Day 1 or Day 0?

Both are used. Counting the day you got together as Day 1 is the most common way for couples, especially in Korea. Day 0 treats the first day as the starting line, like a timer.

If we started on January 1, when is Day 100?

Counting the first day as Day 1, Day 100 is April 10. Counting it as Day 0, Day 100 is April 11.

Does the start time change the count?

No. The day number changes at midnight. A start time only changes the live hours, minutes and seconds.