It's (Christmas) Time for Kotlin

2 min read


By Jørn Ola Birkeland


December 20, 2020

kotlin.time is a small, experimental library introduced with Kotlin 1.3. Its main purpose is to simplify measuring and manipulating elapsed time.

A central class is Duration which is similar to java.time.Duration, but offers more syntactic sugar to simplify operations on elapsed time. In combination with extension properties and operator overloading, you can write things like

val elapsedTime = 28_343_564.nanoseconds + 60.5.milliseconds
val doubleElapsedTime = elapsedTime * 2

which will print 177687us. Another nice feature is the ability to decompose a Duration into convenient parts using toComponents

    val elapsedTime = 3.days + 68.hours + 112.minutes

     elapsedTime.toComponents { hours, minutes, seconds, nanoseconds ->
        println("Hours:        $hours")
        println("Minutes:      $minutes")

However, the library is perhaps most useful if you want to measure the duration of some block of code when executed. measureTime to the rescue

val elapsed: Duration = measureTime {
    println("Measuring time via measureTime")

Or, if you need to return a result from your measured block of code, measureTimedValue is an alternative

val (elapsedTime, returnValue) = measureTimedValue {
    println("Measuring time via measureTime")
    "The returned value"

Under the hood measureTime and measureTimedValue uses the corresponding functions on the underlying TimeSource. By default Monotonic is used, which is a TimeSource Implementation on top of System.nanoTime(), which is the monotonic (strictly increasing) clock in Java, and therefore the most robust way of measuring elapsed time. There is also a TestTimeSource implementation that can be used if you need to control the elapsed time for testing purposes.

TimeSource has a single function, markNow(), which returns a TimeMark. The TimeMark provides methods for checking how much time (as Duration) has passed since the mark, creating offsets from the mark, and checking if a marks has occurred or not.

    val mark = Monotonic.markNow()
    val inThreeSeconds = mark + 3.seconds



kotlin.time is nice, little library which simplifies measuring and manipulating elapsed time compactly and robustly. It is still experimental, and its features can change (and has in the past). But if you are about to sprinkle your Kotlin code with large amounts of java.time.Duration, consider using this little native gem instead.