Kotlin/kotlinx-datetime

kotlinx-datetime

A multiplatform Kotlin library for working with date and time.

See Using in your projects for the instructions on how to set up a dependency in your project.

Design overview

There are a few guiding principles in the design of kotlinx-datetime. First of all, it is pragmatic and focused on the most common problems developers face every day (pun intended) when working with dates and times. It is not all-encompassing and lacks some domain-specific utilities that special-purpose applications might need. We chose convenience over generality, so the API surface this library provides is as minimal as possible to meet the use cases.

The library puts a clear boundary between the physical time of an instant and the local, time-zone-dependent civil time, consisting of components such as year, month, etc that people use when talking about time. We intentionally avoid entities in the library that mix both together and could be misused. However, there are convenience operations that take, for example, a physical instant and perform a calendar-based adjustment (such as adding a month); all such operations explicitly take time-zone information as a parameter to clearly state that their result depends on the civil time zone rules which are subject to change at any time.

The library is based on the ISO 8601 international standard, other ways to represent dates and times are out of its scope. Internationalization (such as locale-specific month and day names) is out of the scope, too.

Types

The library provides a basic set of types for working with date and time:

• LocalDateTime to represent date and time components without a reference to the particular time zone;
• LocalDate to represent the components of date only;
• YearMonth to represent only the year and month components;
• LocalTime to represent the components of time only;
• TimeZone and FixedOffsetTimeZone provide time zone information to convert between kotlin.time.Instant and LocalDateTime;
• Month and DayOfWeek enums;
• DateTimePeriod to represent a difference between two instants decomposed into date and time units;
• DatePeriod is a subclass of DateTimePeriod with zero time components, it represents a difference between two LocalDate values decomposed into date units;
• DateTimeUnit to provide a set of predefined date and time units to use in arithmetic operations on kotlin.time.Instant and LocalDate;
• UtcOffset to represent the amount of time the local datetime at a particular time zone differs from the datetime at UTC;
• LocalDateTimeOffsetInfo to represent an attempt at determining the UTC offset that was in effect when the given wall-clock time was observed.

Type use-cases

Here is some basic advice on how to choose which of the date-carrying types to use in what cases:

• Use kotlin.time.Instant to represent a timestamp of the event that had already happened in the past (like a timestamp of a log entry) or will definitely happen in a well-defined instant of time in the future not far away from now (like an order confirmation deadline in 1 hour from now).

• Use LocalDateTime to represent a time of the event that is scheduled to happen in the far future at a certain local time (like a scheduled meeting in a few months from now). You'll have to keep track of the TimeZone of the scheduled event separately. Try to avoid converting future events to Instant in advance, because time zone rules might change unexpectedly in the future. In this blog post, you can read more about why it's not always a good idea to use Instant everywhere.

  Also use LocalDateTime to decode an Instant to its local datetime components for display and UIs.

• Use LocalDate to represent the date of an event that does not have a specific time associated with it (like a birth date).

• Use YearMonth to represent the year and month of an event that does not have a specific day associated with it or has a day-of-month that is inferred from the context (like a credit card expiration date).

• Use LocalTime to represent the time of an event that does not have a specific date associated with it.

Operations

With the above types, you can get the following operations done.

Converting an instant to local date and time components

An Instant is just a counter of high-resolution time intervals since the beginning of the time scale. To get human readable components from an Instant value, you need to convert it to the LocalDateTime type that represents date and time components without a reference to the particular time zone.

The TimeZone type provides the rules to convert instants from and to datetime components.

val currentMoment: Instant = Clock.System.now()
val datetimeInUtc: LocalDateTime = currentMoment.toLocalDateTime(TimeZone.UTC)
val datetimeInSystemZone: LocalDateTime = currentMoment.toLocalDateTime(TimeZone.currentSystemDefault())

A LocalDateTime instance exposes familiar components of the Gregorian calendar: year, month, day, hour, and so on up to nanosecond. The property dayOfWeek shows what weekday that date is, and dayOfYear shows the day number since the beginning of a year.

Additional time zones can be acquired by their string identifier with the TimeZone.of(id: String) function.

val tzBerlin = TimeZone.of("Europe/Berlin")
val datetimeInBerlin = currentMoment.toLocalDateTime(tzBerlin)

A LocalDateTime instance can be constructed from individual components:

val kotlinReleaseDateTime = LocalDateTime(2016, 2, 15, 16, 57, 0, 0)

Guessing an instant from local date and time components

An instant can be obtained from LocalDateTime by interpreting it as a time moment in a particular TimeZone:

val kotlinReleaseInstant = kotlinReleaseDateTime.toInstant(
    TimeZone.of("Europe/Moscow"),
    TransitionHandler.REJECT_TRANSITIONS
)

Various TransitionHandler values can be used to define what should happen if the date and time components correspond to several instants or none at all.

For fixed-offset time zones, which don't have transitions, transition handlers are not required:

val kotlinReleaseInstant = kotlinReleaseDateTime.toInstant(UtcOffset(hours = 3).asTimeZone())

Getting local date components

A LocalDate represents a local date without time. You can obtain one from an Instant by converting it to LocalDateTime and taking its date property.

val now: Instant = Clock.System.now()
val today: LocalDate = now.toLocalDateTime(TimeZone.currentSystemDefault()).date
// or shorter
val today: LocalDate = Clock.System.todayIn(TimeZone.currentSystemDefault())

Note, that today's date really depends on the time zone in which you're observing the current moment.

LocalDate can be constructed from three components, year, month, and day:

val knownDate = LocalDate(2020, 2, 21)

Getting year and month components

A YearMonth represents a year and month without a day. You can obtain one from a LocalDate by taking its yearMonth property.

val day = LocalDate(2020, 2, 21)
val yearMonth: YearMonth = day.yearMonth

Getting local time components

A LocalTime represents local time without a date. You can obtain one from an Instant by converting it to LocalDateTime and taking its time property.

val now: Instant = Clock.System.now()
val thisTime: LocalTime = now.toLocalDateTime(TimeZone.currentSystemDefault()).time

A LocalTime can be constructed from four components, hour, minute, second, and nanosecond:

val knownTime = LocalTime(hour = 23, minute = 59, second = 12)
val timeWithNanos = LocalTime(hour = 23, minute = 59, second = 12, nanosecond = 999)
val hourMinute = LocalTime(hour = 12, minute = 13)

Converting instant and local datetime to and from the ISO 8601 string

Instant, LocalDateTime, LocalDate and LocalTime provide shortcuts for parsing and formatting them using the extended ISO 8601 format. The toString() function is used to convert the value to a string in that format, and the parse function in companion object is used to parse a string representation back.

val localDateTime = LocalDateTime(2025, 3, 21, 12, 27, 35, 124365453)
localDateTime.toString()  // 2025-03-21T12:27:35.124365453
val sameLocalDateTime = LocalDateTime.parse("2025-03-21T12:27:35.124365453")

LocalDate uses a format with just year, month, and date components, e.g. 2010-06-01.

LocalTime uses a format with just hour, minute, second, and (if non-zero) nanosecond components, e.g. 12:01:03.

LocalDate.parse("2010-06-01")
LocalTime.parse("12:01:03")
LocalTime.parse("12:00:03.999")
LocalTime.parse("12:0:03.999") // fails with an IllegalArgumentException

Working with other string formats

When some data needs to be formatted in some format other than ISO 8601, one can define their own format or use some of the predefined ones:

// import kotlinx.datetime.format.*

val dateFormat = LocalDate.Format {
    monthNumber(padding = Padding.SPACE)
    char('/')
    day()
    char(' ')
    year()
}

val date = dateFormat.parse("12/24 2023")
println(date.format(LocalDate.Formats.ISO_BASIC)) // "20231224"

Using Unicode format strings (like yyyy-MM-dd)

Given a constant format string like the ones used by Java's DateTimeFormatter.ofPattern can be converted to Kotlin code using the following invocation:

// import kotlinx.datetime.format.*

println(DateTimeFormat.formatAsKotlinBuilderDsl(DateTimeComponents.Format {
    byUnicodePattern("uuuu-MM-dd'T'HH:mm:ss[.SSS]Z")
}))

// will print:
/*
date(LocalDate.Formats.ISO)
char('T')
hour()
char(':')
minute()
char(':')
second()
alternativeParsing({
}) {
    char('.')
    secondFraction(3)
}
offset(UtcOffset.Formats.FOUR_DIGITS)
 */

When your format string is not constant, with the FormatStringsInDatetimeFormats opt-in, you can use the format without converting it to Kotlin code beforehand:

val formatPattern = "yyyy-MM-dd'T'HH:mm:ss[.SSS]"

@OptIn(FormatStringsInDatetimeFormats::class)
val dateTimeFormat = LocalDateTime.Format {
    byUnicodePattern(formatPattern)
}

dateTimeFormat.parse("2023-12-24T23:59:59")

Parsing and formatting partial, compound or out-of-bounds data

Sometimes, the required string format doesn't fully correspond to any of the classes kotlinx-datetime provides. In these cases, DateTimeComponents, a collection of all datetime fields, can be used instead.

// import kotlinx.datetime.format.*

val monthDay = DateTimeComponents.Format { monthNumber(); char('/'); day() }
    .parse("12/25")
println(monthDay.day) // 25
println(monthDay.monthNumber) // 12

val dateTimeOffset = DateTimeComponents.Formats.ISO_DATE_TIME_OFFSET
    .parse("2023-01-07T23:16:15.53+02:00")
println(dateTimeOffset.toUtcOffset()) // +02:00
println(dateTimeOffset.toLocalDateTime()) // 2023-01-07T23:16:15.53

Occasionally, one can encounter strings where the values are slightly off: for example, 23:59:60, where 60 is an invalid value for the second. DateTimeComponents allows parsing such values as well and then mutating them before conversion.

val time = DateTimeComponents.Format { time(LocalTime.Formats.ISO) }
    .parse("23:59:60").apply {
        if (second == 60) second = 59
    }.toLocalTime()
println(time) // 23:59:59

Because DateTimeComponents is provided specifically for parsing and formatting, there is no way to construct it normally. If one needs to format partial, complex or out-of-bounds data, the format function allows building DateTimeComponents specifically for formatting it:

DateTimeComponents.Formats.RFC_1123.format {
    // the receiver of this lambda is DateTimeComponents
    setDate(LocalDate(2023, 1, 7))
    hour = 23
    minute = 59
    second = 60
    setOffset(UtcOffset(hours = 2))
} // Sat, 7 Jan 2023 23:59:60 +0200

Instant arithmetic

The Instant arithmetic operations that don't involve the calendar are available purely in the standard library and do not require using kotlinx-datetime:

val now = Clock.System.now()
val instantInThePast: Instant = Instant.parse("2020-01-01T00:00:00Z")
val durationSinceThen: Duration = now - instantInThePast
val equidistantInstantInTheFuture: Instant = now + durationSinceThen

To get the calendar difference between two instants you can use the Instant.periodUntil(Instant, TimeZone) function.

val period: DateTimePeriod = instantInThePast.periodUntil(Clock.System.now(), TimeZone.UTC)

A DateTimePeriod represents a difference between two particular moments as a sum of calendar components, like "2 years, 3 months, 10 days, and 22 hours".

The difference can be calculated as an integer amount of specified date or time units:

val diffInMonths = instantInThePast.until(Clock.System.now(), DateTimeUnit.MONTH, TimeZone.UTC)

There are also shortcuts yearsUntil(...), monthsUntil(...), and daysUntil(...).

A particular amount of datetime units or a datetime period can be added to an Instant with the plus function:

val now = Clock.System.now()
val systemTZ = TimeZone.currentSystemDefault()
val tomorrow = now.plus(2, DateTimeUnit.DAY, systemTZ)
val threeYearsAndAMonthLater = now.plus(DateTimePeriod(years = 3, months = 1), systemTZ)

Note that plus and ...until operations require a TimeZone as a parameter because the calendar interval between two particular instants can be different when calculated in different time zones.

Date arithmetic

Similar operations with date units are provided for LocalDate type:

• LocalDate.plus(number, DateTimeUnit.DateBased)
• LocalDate.plus(DatePeriod)
• LocalDate.until(LocalDate, DateTimeUnit.DateBased) and the shortcuts yearsUntil, monthUntil, daysUntil
• LocalDate.periodUntil(LocalDate): DatePeriod and LocalDate.minus(LocalDate): DatePeriod

Notice that, instead of the general DateTimeUnit and DateTimePeriod types, we're using their subtypes DateTimeUnit.DateBased and DatePeriod respectively. This allows preventing the situations when time components are being added to a date at compile time.

Date + time arithmetic

Arithmetic on LocalDateTime is intentionally omitted. The reason for this is that the presence of daylight saving time transitions (changing from standard time to daylight saving time and back) causes LocalDateTime arithmetic to be ill-defined. For example, consider time gaps (or, as dst tag wiki on Stack Overflow calls them, "spring forward" transitions), that is, ranges of date + time combinations that never occur in a given time zone due to clocks moving forward. If we allowed LocalDateTime arithmetic that ignored time zones, then it could result in LocalDateTime instances that are inside a time gap and are invalid in the implied time zone.

Therefore, the recommended way to use a LocalDateTime is to treat it as a representation of an Instant, perform all the required arithmetic on Instant values, and only convert to LocalDateTime when a human-readable representation is needed.

val timeZone = TimeZone.of("Europe/Berlin")
val localDateTime = LocalDateTime.parse("2021-03-27T02:16:20")
val instant = localDateTime.toInstant(timeZone, TransitionHandler.USE_OFFSET_BEFORE)

val instantOneDayLater = instant.plus(1, DateTimeUnit.DAY, timeZone)
val localDateTimeOneDayLater = instantOneDayLater.toLocalDateTime(timeZone)
// 2021-03-28T03:16:20, as 02:16:20 that day is in a time gap

val instantTwoDaysLater = instant.plus(2, DateTimeUnit.DAY, timeZone)
val localDateTimeTwoDaysLater = instantTwoDaysLater.toLocalDateTime(timeZone)
// 2021-03-29T02:16:20

Implementation

The implementation of datetime types, such as LocalDateTime, TimeZone, and so on, relies on:

• in JVM: java.time API;
• the other platforms: based on the ThreeTen backport project
  • time zone support on JS and Wasm/JS is provided by the js-joda library.

Known/open issues, work TBD

• Flexible locale-neutral parsing and formatting facilities are needed to support various datetime interchange formats that are used in practice (in particular, various RFCs).

Using in your projects

Note that the library is experimental, and the API is subject to change.

The library is published to Maven Central.

The library is compatible with the Kotlin Standard Library not lower than 2.3.21.

If you target Android devices running below API 26, you need to use Android Gradle plugin 4.0 or newer and enable core library desugaring.

Deprecation of Instant

kotlinx-datetime versions earlier than 0.7.0 used to provide kotlinx.datetime.Instant and kotlinx.datetime.Clock. The Kotlin standard library started including its own, identical kotlin.time.Instant and kotlin.time.Clock, as it became evident that Instant was also useful outside the datetime contexts.

Here is the recommended procedure for migrating from kotlinx-datetime version 0.6.x or earlier to 0.7.x:

• First, simply try upgrading to 0.7.1. If your project has a dependency on kotlinx-datetime, but doesn't have dependencies on other libraries that are themselves reliant on an older kotlinx-datetime, you are good to go: the code should compile and run. This applies both to applications and to libraries!
• If your project depends on other libraries that themselves use an older version of kotlinx-datetime, then your code may fail at runtime with a ClassNotFoundException for kotlinx.datetime.Instant or kotlinx.datetime.Clock, or maybe even fail to compile. In that case, please check if the affected libraries you have as dependencies have already published a new release adapted to use Instant and Clock from kotlin.time.
• If you use kotlinx-serialization to serialize the Instant type, update that dependency to use 1.9.0 or a newer version.
• If all else fails, use the compatibility release of kotlinx-datetime. Instead of the version 0.7.1, use 0.7.1-0.6.x-compat. This artifact still contains kotlinx.datetime.Instant and kotlinx.datetime.Clock, ensuring that third-party libraries reliant on them can still be used. This artifact is less straightforward to use than 0.7.1, so only resort to it when libraries you don't control require that the removed classes still exist.

Tips for fixing compilation errors:

• If you encounter resolution ambiguity errors for Instant or Clock, see if you have import kotlin.time.* along with import kotlinx.datetime.*. Since both libraries have a Clock and an Instant, you have to manually add import kotlin.time.Instant and/or import kotlin.time.Clock explicitly.
• When using the compatibility release of kotlinx-datetime, you may encounter errors like "required kotlinx.datetime.Instant, found kotlin.time.Instant" or vice versa.
  • First, please check if you have imported a kotlinx.datetime class when a kotlin.time class would work. The final goal is getting rid of kotlinx.datetime.Instant, so limit its usage as much as possible!
  • If you have no choice but to use an Instant or Clock from kotlinx-datetime (for example, because a third-party library accepts a kotlinx.datetime.Instant as a parameter or returns it as a function result), you can use the compatibility functions:
    • kotlin.time.Instant.toDeprecatedInstant(): kotlinx.datetime.Instant
    • kotlin.time.Clock.toDeprecatedClock(): kotlinx.datetime.Clock
    • kotlinx.datetime.Instant.toStdlibInstant(): kotlin.time.Instant
    • kotlinx.datetime.Clock.toStdlibClock(): kotlin.time.Clock

Compatibility releases will be published for versions up to 0.8.x of kotlinx-datetime, but not longer.

Gradle

• Add the Maven Central repository if it is not already there:

repositories {
    mavenCentral()
}

• In multiplatform projects, add a dependency to the dependencies of the commonMain source set

kotlin {
    sourceSets {
        commonMain {
             dependencies {
                 implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.8.0")
             }
        }
    }
}

• To use the library in a single-platform project, add a dependency to the dependencies block.

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.8.0")
}

Note about time zones in JS

By default, there's only one time zone available in Kotlin/JS: the SYSTEM time zone with a fixed offset.

If you want to use all time zones in the Kotlin/JS platform, you need to add the following npm dependency (replace X.X.X below with the latest version from here):

kotlin {
    sourceSets {
        val jsMain by getting {
            dependencies {
                implementation(npm("@js-joda/timezone", "X.X.X"))
            }
        }
    }
}

and after that add the following initialization code to your project:

@JsModule("@js-joda/timezone")
@JsNonModule
external object JsJodaTimeZoneModule

@OptIn(ExperimentalJsExport::class)
@JsExport
val jsJodaTz = JsJodaTimeZoneModule

This code can be placed in any file in the Kotlin/JS source set.

Note about time zones in Wasm/JS

Wasm/JS uses the same time zone support as JS, so almost the same instructions apply.

In your Gradle build script, add the following dependency (replace X.X.X below with the latest version from here):

kotlin {
    sourceSets {
        val wasmJsMain by getting {
            dependencies {
                implementation(npm("@js-joda/timezone", "X.X.X"))
            }
        }
    }
}

and after that add the following initialization code in your project:

@JsModule("@js-joda/timezone")
external object JsJodaTimeZoneModule

private val jsJodaTz = JsJodaTimeZoneModule

Note about time zones in Wasm/WASI

By default, there's only one time zone available in Kotlin/Wasm WASI: the UTC time zone with a fixed offset.

If you want to use all time zones in Kotlin/Wasm WASI platform, you need to add the following dependency:

kotlin {
    sourceSets {
        val wasmWasiMain by getting {
            dependencies {
                implementation("kotlinx-datetime-zoneinfo", "2026c-spi.0.8.0")
            }
        }
    }
}

Maven

Add a dependency to the <dependencies> element. Note that you need to use the platform-specific -jvm artifact in Maven.

<dependency>
    <groupId>org.jetbrains.kotlinx</groupId>
    <artifactId>kotlinx-datetime-jvm</artifactId>
    <version>0.8.0</version>
</dependency>

Building

The project requires JDK 17 to build classes and to run tests. Gradle will try to find it among the installed JDKs or download it automatically if it couldn't be found. The path to JDK 17 can be additionally specified with the environment variable JDK_17_0. For local builds, you can use a later version of JDK if you don't have that version installed. Specify the version of this JDK with the java.mainToolchainVersion Gradle property.

After that, the project can be opened in IDEA and built with Gradle.

For building and running benchmarks, see README.md