How about birthdays? When you turn "1" it's your second birthday. Or counting three seconds. Start counting zero..one..two..three.
The number on your yearly "birthday" is not the number of birthdays you had (you only have one birthday) but the number of years you survived since your birth.
So the day of your fifth birthday is the first day of your sixth year alive.
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Korea has a system similar to this. You start at age 1 and get one year older each New Year's Day.
https://www.90daykorean.com/korean-age-all-about-age-in-kore...
They changed the system last year after using it for many years (+/-1).
https://www.aljazeera.com/amp/news/2023/6/28/why-are-south-k...