63. Date & Time

On the JVM, Kotlin uses the java.time package. Its types are immutable, so methods like plusDays return a new value rather than mutating in place. We use fixed dates here so the output is deterministic — in real code you would reach for LocalDate.now().

import java.time.LocalDate
import java.time.LocalDateTime
import java.time.Duration
import java.time.Period
import java.time.format.DateTimeFormatter

fun main() {
    val date = LocalDate.of(2024, 1, 15)
    println(date)
    println(date.plusDays(20))           // immutable: returns a new date

    val launch = LocalDateTime.of(2024, 1, 15, 9, 30)
    val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm")
    println(launch.format(formatter))
}

Period measures a span in years/months/days between two dates, while Duration measures a time-based amount in hours/minutes/seconds.

val period = Period.between(LocalDate.of(2024, 1, 1), LocalDate.of(2024, 3, 15))
println("${period.months} months, ${period.days} days")

val duration = Duration.ofHours(2).plusMinutes(30)
println("${duration.toMinutes()} minutes")

Kotlin also has its own multiplatform kotlin.time.Duration, created with extensions like 2.hours and 30.minutes; it is independent of java.time and works on every target.

Running it:

$ kotlin run
2024-01-15
2024-02-04
2024-01-15 09:30
2 months, 14 days
150 minutes

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