How to calculate age from a birth date

Short answer

Period.between() from the birth date to today gives the number of full years. In the example "today" is fixed so the result is always the same:

void main() {
    var birthday = LocalDate.of(1990, 7, 15);
    var today = LocalDate.of(2026, 3, 8);
 
    IO.println(Period.between(birthday, today).getYears());
}

Output:

In real code use LocalDate.now().

With ChronoUnit

ChronoUnit.YEARS.between() returns the same as a single number:

void main() {
    var birthday = LocalDate.of(1990, 7, 15);
 
    IO.println(ChronoUnit.YEARS.between(birthday, LocalDate.of(2026, 7, 14)));
    IO.println(ChronoUnit.YEARS.between(birthday, LocalDate.of(2026, 7, 15)));
}

Output:

35
36

The full age

A Period also holds months and days:

void main() {
    var age = Period.between(LocalDate.of(2024, 1, 20), LocalDate.of(2026, 3, 8));
    IO.println(age.getYears() + " y " + age.getMonths() + " m " + age.getDays() + " d");
}

Output:

2 y 1 m 16 d

Common mistake

Subtracting years: 2026 - 1990. If the birthday has not happened yet this year, the age comes out one year too high:

void main() {
    var birthday = LocalDate.of(1990, 7, 15);
    var today = LocalDate.of(2026, 3, 8);
 
    IO.println(today.getYear() - birthday.getYear());
    IO.println(Period.between(birthday, today).getYears());
}

Output:

36
35

Thanks for reading!

Short answer

With ChronoUnit

The full age

Common mistake

Functions used in this recipe

Thanks for reading!