How to Calculate Age in Kotlin (LocalDate and Period Examples)

Calculating age sounds easy until you hit birthdays that have not happened yet this year, leap years, and mixed date formats. In Kotlin, the safest approach is usually to rely on java.time.

Fast option: use an online age calculator

If you need an exact answer in years, months, and days, or you want a quick check for test cases, use a tool first and then mirror the logic in code.

Open the age calculator and enter the date of birth plus the reference date.

Calculate age with LocalDate and Period

This is the cleanest Kotlin solution for most apps.

import java.time.LocalDate
import java.time.Period

fun ageYears(dateOfBirth: LocalDate, asOf: LocalDate = LocalDate.now()): Int {
    return Period.between(dateOfBirth, asOf).years
}

val years = ageYears(LocalDate.parse("1998-10-12"))

Why this works well

Period.between() understands calendar dates, so you avoid manual month and day comparisons. That makes it safer than subtracting years directly.

Parse a string safely

If your input is a string, parse it into LocalDate first.

import java.time.LocalDate

val dob = LocalDate.parse("1998-10-12")
val age = ageYears(dob)

Custom format example

If the input is not ISO format, add a formatter.

import java.time.LocalDate
import java.time.format.DateTimeFormatter

val formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy")
val dob = LocalDate.parse("12/10/1998", formatter)
val age = ageYears(dob)

FAQ

Should I use LocalDate or LocalDateTime?

Use LocalDate for age based on birthdays. Age is usually a calendar concept, not a timestamp concept.

What about leap day birthdays?

LocalDate and Period handle calendar rules correctly, which is another reason to prefer them over manual math.

How do I get years, months, and days?

Use the full Period instead of just .years.

val diff = Period.between(dob, LocalDate.now())
println("${diff.years} years, ${diff.months} months, ${diff.days} days")