I have a Date - Speaker Deck

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I have a Date T renaud_mathieu

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What day is it Today?

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Problem #1 Different calendars…

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Current year according to various historical and world calendars, as of January 1. 2023 0 1500 3000 4500 6000 Hebrew Chinese Gregorian Islamic French Revolutionary 230 1   2   4   5  

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Problem #2 Different timezones…

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https://data.iana.org/time-zones/tzdb/NEWS

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https://data.iana.org/time-zones/tzdb/NEWS

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Problem #3 The rotation of the Earth is not regular…

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UTC Coordinated Universal Time UTC Coordinated Universal Time UT Universal Time TAI International Atomic Time

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UTC Français TUC Temps universel coordonné Anglais CUT Coordinated Universal Time 🤝 UTC Universal Time Coordinated Universel Temps Coordonné Coordinated Universal Time

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UTC UTC Coordinated Universal Time GMT Greenwich Mean Time ≠ Coordinated Universal Time

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🛠 Tools

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MACHINE TIME MANAGEMENT HUMAN TIME MANAGEMENT

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🛠 Unix Epoch • Unix time is a date and time representation widely used in computing. • Leap seconds* are known only 6 months in advance. • Year 2038 problem *leap seconds = secondes intercalaires 🇫🇷 MACHINE TIME MANAGEMENT

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2023-04-27T15:50:00+02:00 General Principles 🛠 ISO 8601 MACHINE TIME MANAGEMENT HUMAN TIME MANAGEMENT 🤝

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2023-04-27T15:50:00+02:00 YYYY-MM-DD or YYYYMMDD YYYY-MM (but not YYYYMM) Thh:mm:ss.sss or Thhmmss.sss Thh:mm:ss or Thhmmss Thh:mm or Thhmm Thh Z ±hh:mm ±hhmm ±hh -YY-MM or -YYMM —MM-DD or —MMDD - - MM - - - DD

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History of Time, in a Java world…

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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java.util

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java.util.Date import java.util.Date fun main() { val date = Date().apply { year = 2023 month = 4 day = 27 } print(date) } / / Fri May 04 21:18:05 UTC 3923

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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Joda Time & ThreeTen

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Joda Time A Powerful and Versatile Date and Time Library • Provide an alternative to the JDK date management classes which have several drawbacks. • Aimed to provide a simpler and more intuitive API for handling dates and times.

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ThreeTenBP ThreeTenABP

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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java.time

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MACHINE TIME MANAGEMENT HUMAN TIME MANAGEMENT

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MACHINE TIME MANAGEMENT

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Machine Time Instant • Encapsulates a point in time with nanosecond precision. • Represent a point in the timeline. • The Instant class is the closest class to the java.util.Date class I MACHINE TIME MANAGEMENT

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Machine Time Instant import java.time.Instant fun main() { println(Instant.EPOCH) println(Instant.now()) } —— 1970-01-01T00:00:00Z 2023-04-26T09:21:39.149061Z MACHINE TIME MANAGEMENT

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Machine Time Duration • Encapsulates a time between two instants stored with nanosecond precision. • Duration can be negative! • The ISO-8601 standard de fi nes the textual format of a duration: PTnHnMn.nS MACHINE TIME MANAGEMENT

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Machine Time Duration D I I MACHINE TIME MANAGEMENT

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import java.time.Instant import java.time.Duration import java.time.temporal.ChronoUnit fun main() { var oneHour = Duration.ofHours(1L) println("oneHour = $oneHour") oneHour = Duration.ofMinutes(60L) println("oneHour = $oneHour") oneHour = Duration.ofSeconds(3600L) println("oneHour = $oneHour") val instant1 = Instant.parse("2023-04-26T23:59:59Z") val instant2 = instant1.plus(1L, ChronoUnit.DAYS) val duration = Duration.between(instant1, instant2) println("duration = $duration") } oneHour = PT1H oneHour = PT1H oneHour = PT1H duration = PT24H

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MACHINE TIME MANAGEMENT HUMAN TIME MANAGEMENT

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HUMAN TIME MANAGEMENT

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LocalDate LocalTime LocalDateTime Period Duration LocalDate private final int year; private final short month; private final short day; val date1 = LocalDate.now() val date2 = LocalDate.of(1987, Month.JULY, 6) val date3 = LocalDate.ofEpochDay(113) println(date1) println(date2) println(date3) —— 2023-07-05 1987-07-06 1970-04-24

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LocalDate LocalTime LocalDateTime Period Duration val time1 = LocalTime.now() val time2 = LocalTime.of(23,59,59,104534) println(time1) println(time2) println(time1.minusHours(1)) —— 10:06:27.256581 23:59:59.000104534 09:06:27.256581 LocalTime private final byte hour; private final byte minute; private final byte second; private final int nano;

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LocalDate LocalTime LocalDateTime Period Duration val date = LocalDate.now() val time = LocalTime.now() val dateTime1: LocalDateTime = date.atTime(time) val dateTime2 = time.atDate(date) println(dateTime1) println(dateTime2) println(dateTime1.dayOfWeek) —— 2023-07-04T15:42:32.878386241 2023-07-04T15:42:32.878386241 TUESDAY LocalDateTime private final LocalDate date; private final LocalTime time; LocalTime LocalDate

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LocalDate LocalTime LocalDateTime Period Duration val date = LocalDate.now().minusWeeks(2) val dateTime = LocalDateTime.now() val period = Period.between(date,dateTime.toLocalDate()) println(date) println(dateTime) println(period) —— 2023-06-20 2023-07-04T15:43:57.093814609 P14D Period private final int years; private final int months; private final int days;

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LocalDate LocalTime LocalDateTime Period Duration val time1 = LocalTime.now() val time2 = LocalTime.now().minusMinutes(40) val duration = Duration.between(time1, time2) println(duration) —— PT-39M-59.999147S Duration private final long seconds; private final int nanos;

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Time Zones ZoneId • Encapsulates a time zone identi fi ed by its unique id: « Europe/Paris » • Contains rules to transform Instant to LocalDateTime

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Time Zones ZoneId + ZoneRules • There are three types of IDs: • simple: corresponds to the one encapsulated in a ZoneOffset. Its representation begins with the letter Z then the sign + or - and the offset value • offset-style: starts with a pre fi x (UTC, GMT or UT) then the + or - sign and the offset value • region-based: starts with at least two characters different from the offset-style pre fi xes and the characters + and -,

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val zoneId: ZoneId = ZoneId.of("Europe/Paris") val rules = zoneId.getRules() println(rules.getOffset(LocalDateTime.of(2023, Month.MARCH,26, 0, 0))) println(rules.getOffset(LocalDateTime.of(2023, Month.MARCH, 27, 0, 0))) +01:00 +02:00

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ZonedDateTime vs OffsetDateTime • ZonedDateTime : • is based on the time zone identi fi er, such as « America/New_York » • is used when you need to represent date and time information in different time zones • OffsetDateTime: • is based on the offset from UTC, such as « +03:00 ». • is used when you need to represent date and time information with a known UTC offset.

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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History of Time, in a Java world… 1996 Java 1.0 java.util.date 1997 Java 1.1 java.util.calendar 2005 joda-time 2014 JSR-310 2015 ThreeTenABP 2020 kotlinx-datetime

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kotlinx-datetime

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kotlinx-datetime Bene fi ts public fun DateTimePeriod( years: Int = 0, months: Int = 0, days: Int = 0, hours: Int = 0, minutes: Int = 0, seconds: Int = 0, nanoseconds: Long = 0 ): DateTimePeriod = buildDateTimePeriod(totalMonths(years, months), days, totalNanoseconds(hours, minutes, seconds, nanoseconds)) • It's in the alpha stage, and the development continues. • Multiplatform support • @Serializable • Simpler API

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kotlinx-datetime Bene fi ts • It's in the alpha stage, and the development continues. • Multiplatform support • @Serializable • Simpler API // Support ISO-8601 format "2010-06-01T22:19:44.475Z".toInstant() "2010-06-01T22:19:44".toLocalDateTime() "2010-06-01".toLocalDate() "12:01:03".toLocalTime() "12:0:03.999".toLocalTime()

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How about Today?

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Project Mainline Modular System Components IANA releases an update to the   Time Zone Database Google or the Android partner prepares a Time Zone Data module update (APEX fi le) End-user device downloads the update, reboots, then applies the changes

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How about today? JSR-310 (API) TimeZone Management Multiplatform java.util joda-time-android ~ ThreeTenABP X ~ java.time X X* kotlinx.datetime X* X* X

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Thank you! www.renaudmathieu.com