Count today as day 1 or day 0?
Short answer: for anniversaries and couple days, count the start date as day 1. For countdowns, don't. That one choice is why a 100th-day date can land one day apart in two apps.
1. Day 1 counting (anniversaries)
In Korea, couples count the day they started dating as day 1. So the 100th day falls 99 days after the start date: start on January 1, and day 100 is April 10 (non-leap year). Many baby "days since birth" trackers use the same rule.
2. Day 0 counting (elapsed days)
Plain "days since" counts how many full days have passed. On the start date itself it is 0, the next day is 1. With this rule, day 100 is April 11.
3. Countdowns
Countdowns count days remaining: D-1 the day before, D-Day the day itself. The day-1 switch does not apply.
Doing it with Nearday
Nearday has a "count today as day 1" switch per event. Turn it on for an anniversary, leave it off for trips and exams. Past events keep counting up, and reminders can fire on the day or 1, 3, 7 or 30 days before.
FAQ
Is the first day of dating day 1 or day 0?
In Korea, couple days are counted with the first day as day 1, so the 100th day is 99 days after the start date.
Which setting should I use for a countdown?
Leave count-today-as-day-1 off. Countdowns count the days remaining, not the days elapsed.
Why is my 100-day date one day off?
Your app counts the start day as day 0. Turn on count today as day 1.