The time relativity principle

The moment when you code. You type. You type a lot. You type `LocalDateTime`. The moment you know you are doomed. How hard can time be? Sometimes, time gives us a hard time. This time, we will have a deeper dive into what it takes for a String of `2019-05-15T09:00:00` to become an instance of `java.time.Instant`. This is what this lightning talk will be about.

Transcript

1. The time relativity
   principle
   Jakub Marchwicki <@kubem>
   

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2. Software engineer
   Consultant
   Trainer
   Chief Mob Officer
   Jakub Marchwicki <@kubem>
   http://jakub.marchwicki.pl
   

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3. 
   1023
   1
   1899-12-30T07:20:00
   
   

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4. 
   1023
   1
   1899-12-30T07:20:00
   
   

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5. 
   106
   131
   
   
   1017
   0
   1899-12-30T07:18:00
   
   
   1023
   1
   1899-12-30T07:20:00
   
   
   
   

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6. 
   106
   131
   
   
   1017
   0
   1899-12-30T07:18:00
   
   
   1023
   1
   1899-12-30T07:20:00
   
   
   
   

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7. Why 1899-12-30T07:20:00?
   jakub marchwicki <@kubem>
   

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8. What is time “zero”?
   https://en.wikipedia.org/wiki/Epoch_(computing)
   jakub marchwicki <@kubem>
   

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9. What is time “zero”?
   January 1, 1970 - Linux, Java, JavaScript, PHP
   https://en.wikipedia.org/wiki/Epoch_(computing)
   jakub marchwicki <@kubem>
   

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10. What is time “zero”?
    January 1, 1970 - Linux, Java, JavaScript, PHP
    January 1, 2000 - PostgreSQL
    January 1, 2001 - Apple Cocoa
    https://en.wikipedia.org/wiki/Epoch_(computing)
    jakub marchwicki <@kubem>
    

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11. What is time “zero”?
    January 1, 1 - Microsoft .NET
    January 1, 1601 - COBOL
    January 1, 1900 - Common Lisp
    January 1, 1970 - Linux, Java, JavaScript, PHP
    January 1, 2000 - PostgreSQL
    January 1, 2001 - Apple Cocoa
    https://en.wikipedia.org/wiki/Epoch_(computing)
    jakub marchwicki <@kubem>
    

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12. What is time “zero”?
    January 1, 1 - Microsoft .NET
    January 1, 1601 - COBOL
    December 30, 1899 - Microsoft COM DATE
    January 1, 1900 - Common Lisp
    January 1, 1970 - Linux, Java, JavaScript, PHP
    January 1, 2000 - PostgreSQL
    January 1, 2001 - Apple Cocoa
    https://en.wikipedia.org/wiki/Epoch_(computing)
    jakub marchwicki <@kubem>
    

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13. http://www.odi.ch/prog/design/datetime.php
    FALSEHOOD
    Epoch is the universal representation of
    beginning of time in computer systems
    jakub marchwicki <@kubem>
    

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14. Let’s convert - naive approach
    minOccurs="1" maxOccurs="1" type="s:dateTime" />
    @XmlElement(name = "PassingTime", required = true)
    @XmlSchemaType(name = "dateTime")
    protected XMLGregorianCalendar passingTime;
    jakub marchwicki <@kubem>
    

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15. Let’s convert - the quiz
    var date = DatatypeFactory.newInstance()
    .newXMLGregorianCalendar("1899-12-30T07:20:00")
    .toGregorianCalendar().toInstant()
    A. 07:20 B. 07:44
    jakub marchwicki <@kubem>
    Point in time, epoch-seconds measured
    from the standard Java epoch of
    1970-01-01T00:00:00Z
    As we have number of seconds
    measured from UTC, we need to
    say which timezone we are in
    LocalDateTime.ofInstant(date, ZoneId.of("Europe/Warsaw"))
    .toLocalTime()
    C. 07:04 D. 06:20
    jshell> LocalDateTime.of(1899, 12, 30, 7, 20).toInstant(UTC).getEpochSecond()
    $8 ==> -2209135200
    

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16. Let’s convert - the quiz
    var date = DatatypeFactory.newInstance()
    .newXMLGregorianCalendar("1899-12-30T07:20:00")
    .toGregorianCalendar().toInstant()
    LocalDateTime.ofInstant(date, ZoneId.of("Europe/Warsaw"))
    .toLocalTime()
    A. 07:20 B. 07:44
    C. 07:04 D. 06:20
    

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17. What is the timezone for Europe/Warsaw
    CET (UTC+1h) / CEST (UTC+2h)
    https://en.wikipedia.org/wiki/Epoch_(computing)
    jakub marchwicki <@kubem>
    

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18. UTC+1h - 1965 - 1976
    UTC+1h / UTC+2h - 1957 - 1964
    …
    UTC+1:24h - 1800 - 1914
    What is the timezone for Europe/Warsaw
    CET (UTC+1h) / CEST (UTC+2h) - since 1977
    https://en.wikipedia.org/wiki/Epoch_(computing)
    jakub marchwicki <@kubem>
    

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19. http://www.odi.ch/prog/design/datetime.php
    FALSEHOOD
    My timezone will never change
    jakub marchwicki <@kubem>
    

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20. time zone rules change, and that applications
    should expect that they will change.
    This isn’t a corner case,
    it’s the normal way things work
    https://codeblog.jonskeet.uk/2019/03/27/storing-utc-is-not-a-silver-bullet/
    TAKEAWAY
    jakub marchwicki <@kubem>
    

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21. Let’s pretend you are Samoan
    

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23. samoa
    

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24. samoa
    international
    date line
    

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25. samoa
    international
    date line
    

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27. UTC +12:00
    UTC -11:00
    

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28. The rule of the game for programming with time
    is that you should opt to go as simple as
    possible as soon as possible.
    https://zachholman.com/talk/utc-is-enough-for-everyone-right
    TAKEAWAY
    jakub marchwicki <@kubem>
    

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29. Let’s convert
    var xmlGregoriaCalendar = DatatypeFactory.newInstance()
    .newXMLGregorianCalendar("1899-12-30T07:20:00")
    LocalDateTime.parse(xmllGregoriaCalendar.toXmlFormat())
    .toLocalTime()
    jakub marchwicki <@kubem>
    

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30. http://www.odi.ch/prog/design/datetime.php
    FALSEHOOD
    A wall clock time everything
    jakub marchwicki <@kubem>
    

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31. Don’t be like the Russian Olympic shooting team
    The Russian Olympic shooting team was proud to head to London for the
    Olympic Games in 1908
    They saved the date, hit the road
    And arrived to London two days too late. Missed their game
    Great Britain was on Gregorian
    Calendar since 1752. Poland
    since 1582 (first to adopt)
    Russian adopted Gregorian
    Calendar in 1918. Switched from
    Julian Calendar, changing
    1st February 1918 to 14th of Feb.
    

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32. http://www.odi.ch/prog/design/datetime.php
    FALSEHOOD
    UTC all the things
    jakub marchwicki <@kubem>
    

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33. 
    124
    219
    
    
    114
    0
    1899-12-30T03:48:00
    
    
    1023
    1
    1899-12-30T03:51:00
    
    
    
    
    124
    219
    
    
    114
    0
    1899-12-30T03:48:00Z
    
    
    1023
    1
    1899-12-30T03:51:00
    
    
    
    
    124
    219
    
    
    114
    0
    228
    
    
    1023
    1
    1899-12-30T03:51:00
    
    
    
    
    124
    219
    
    
    114
    0
    PT3H48M
    
    
    1023
    1
    1899-12-30T03:51:00
    
    
    
    Why can’t we do just LocalTime
    

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34. 
    149
    112
    
    
    114
    0
    1899-12-30T03:48:00
    
    
    
    
    
    622
    100
    
    
    
    1963
    43
    1899-12-31T04:26:00
    
    
    
    

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35. Let’s convert
    minOccurs="1" maxOccurs="1" type="s:long" />
    protected Long passingTime;
    LocalDateTime.of(
    LocalDate.of(2019, 11, 07),
    LocalTime.MIDNIGHT.plus(Duration.ofMinutes(passingTime))
    )
    jakub marchwicki <@kubem>
    

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36. https://codeblog.jonskeet.uk/2019/03/27/storing-utc-is-not-a-silver-bullet/
    TAKEAWAY
    it depends!
    storing UTC is not always the wrong thing to do,
    but it’s not always the right thing to do either.
    jakub marchwicki <@kubem>
    

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37. Thank you!
    https://speakerdeck.com/kubamarchwicki/the-time-relativity-principle
    Please vote and rate the session
    

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38. Dates : Time : Timezones :: Links
    ● There is an ingenious talk by Tomek Nurkiewicz on timezones
    http://nurkiewicz.github.io/talks/confitura2013/
    ● Awesome falsehood on time & timezones
    https://github.com/kdeldycke/awesome-falsehood#dates-and-time
    ● Canvas for this talk was an actual problem
    https://stackoverflow.com/questions/54326076/timezone-inconsistenci
    es-on-converting-xmlgregoriancalendar-to-localdatetime
    ● No timezones. Ever!
    https://en.wikipedia.org/wiki/International_Fixed_Calendar
    ● These slides
    https://speakerdeck.com/kubamarchwicki/the-time-relativity-principle
    Jakub Marchwicki <@kubem>
    

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