kotlinx-datetime

A multiplatform Kotlin library for working with date and time.

See Using in your projects for the instructions how to setup 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, 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 physical time of an instant and a 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 operation explicitly take a time-zone information as 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 the scope, too.

Types

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

• Instant to represent a moment on the UTC-SLS time scale;
• Clock to obtain the current instant;
• LocalDateTime to represent date and time components without a reference to the particular time zone;
• LocalDate to represent the components of date only;
• TimeZone and FixedOffsetTimeZone provide time zone information to convert between 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 provides a set of predefined date and time units to use in arithmetic operations on Instant and LocalDate.
• UtcOffset represents the amount of time the local date/time at a particular time zone differs from the date/time at UTC.

Type use-cases

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

• Use 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 date-time components for display and UIs.

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

Operations

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

Getting the current moment of time

The current moment of time can be captured with the Instant type. To obtain an Instant corresponding to the current moment of time, use now() function of the Clock interface:

val clock: Clock = ...
val currentMoment = clock.now()

An instance of Clock can be injected through the function/class parameters, or you can use its default implementation Clock.System that represents the system clock:

val currentMoment = Clock.System.now()

Converting an instant to local date and time components

Instant is just a counter of high resolution time intervals since the beginning of time scale. To get human readable components from an Instant value you need to convert it to 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 date/time components.

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

LocalDateTime instance exposes familiar components of the Gregorian calendar: year, month, dayOfMonth, 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)

LocalDateTime instance can be constructed from individual components:

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

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

val kotlinReleaseInstant = kotlinReleaseDateTime.toInstant(TimeZone.of("UTC+3"))

Getting local date components

LocalDate type represents local date without time. You can obtain it from 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 more short
val today: LocalDate = Clock.System.todayAt(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)

Converting instant to and from unix time

An Instant can be converted to a number of milliseconds since the Unix/POSIX epoch with the toEpochMilliseconds() function. To convert back, use Instant.fromEpochMilliseconds(Long) companion object function.

Converting instant and local date/time to and from string

Currently, Instant, LocalDateTime, and LocalDate only support 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 instantNow = Clock.System.now()
instantNow.toString()  // returns something like 2015-12-31T12:30:00Z
val instantBefore = Instant.parse("2010-06-01T22:19:44.475Z")

Alternatively, String.to...() extension functions can be used instead of parse, where it feels more convenient:

LocalDateTime uses the similar format, but without Z UTC time zone designator in the end.

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

"2010-06-01T22:19:44.475Z".toInstant()
"2010-06-01T22:19:44".toLocalDateTime()
"2010-06-01".toLocalDate()

Instant arithmetic

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

Duration is a type from the experimental kotlin.time package in the Kotlin standard library. This type holds the amount of time that can be represented in different time units: from nanoseconds to 24H days.

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

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

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 date/time units or a date/time 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 TimeZone as a parameter because the calendar interval between two particular instants can be different, when calculated in different time zones.

Date arithmetic

The 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 general DateTimeUnit and DateTimePeriod 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)

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 date/time types, such as Instant, LocalDateTime, TimeZone and so on, relies on:

• in JVM: java.time API;
• in JS: js-joda library;
• in Native: based on ThreeTen backport project
  • time zone support is provided by date C++ library;

Known/open issues, work TBD

• Some kind of Clock interface is needed as a pluggable replacement for Instant.now().
• Flexible locale-neutral parsing and formatting facilities are needed to support various date/time 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 1.5.0.

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.

Gradle

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

repositories {
    mavenCentral()
}

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

kotlin {
    sourceSets {
        commonMain {
             dependencies {
                 implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.3.2")
             }
        }
    }
}

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

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-datetime:0.3.2")
}

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 Kotlin/JS platform, you need to add the following npm dependency:

kotlin {
    sourceSets {
        val jsMain by getting {
            dependencies {
                implementation(npm("@js-joda/timezone", "2.3.0"))
            }
        }
    }
}

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

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

private val jsJodaTz = JsJodaTimeZoneModule

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>0.3.2</version>
</dependency>

Building

Before building, ensure that you have thirdparty/date submodule initialized and updated. IDEA does that automatically when cloning the repository, and if you cloned it in the command line, you may need to run additionally:

git submodule init
git submodule update

The project requires JDK 8 to build classes and to run tests. Gradle will try to find it among the installed JDKs or provision it automatically if it couldn't be found. The path to JDK 8 can be additionally specified with the environment variable JDK_8. 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.