1. Joda Time and a Brief History of the World
Daniel
Joda Time and a Brief History of the World

2. Preamble

3. Epoch (reference date)
Serves as a reference point from which time is measured.
Most of the world uses Anno Domini calendar era (the civil calendar era used internationally and in many countries)
Traditionally-reckoned Incarnation of Jesus.
Many other current and historical calendar eras exist, each with its own epoch.

4. Asian Epochs Japan: From the accession of the current emperor. China:
Followed same with emperor until 1912.
Since 1912: Republican era was established still used in Taiwan
Common Era: People's republic of China starts in 1949.
India: The Saka Era. Vernal Equinox since year 78.
North Korea: Starts 1912: but because Kim Il-Sung was born on that date.
Thailand: Founding of Bangkok in 1782, but in 1941 uses 543BC started Thai Solar Calendar.

5. Religious Epochs
Israel - Hebrew Calendar dates from "Creation" (Anno Mundi) / March 18, 3952 BC
In case you are wondering Earth Day is on Spring Equinox
Islam - Anno Hegiræ - The year of the Hijra (the Migration of Mohammed to Medina in 622
Bikram Samwat
Calendar established by Indian emperor Vikramaditya.
It is a popularly used calendar in India and the official calendar of Bangladesh and Nepal.

6. Already it's complex

7. Computer System Epochs
Matlab, Turbo DB, tdbengine – January 1, 0
Symbian, .NET, RXX, Rata Die – January 1, 1
NTFS, COBOL, Win32/64 – January 1, 1601
MS SQL Server – January 1, 1753
Microsoft Excel, Lotus January 0, 1900.
Common LISP, CICS, Network Time Protocol – January 1, 1900
LabView, Mac OS <= 9.0, Palm – January 1, 1904
And More...

8. Perhaps a Standard 0 = January 1, 1970 Midnight GMT. Unix Time
Unix, Linux, Ruby, Mac OS X, Java, JavaScript, C, PHP

9. GMT Greenwich Mean Time
Mean solar time at the Royal Observatory in Greenwich, London.
Britain had been the world's time keeper since Victorian Age.
It is commonly used in practice to refer to UTC (not accurate)
Side Note: GMT used in winter, British Summer Time in summer

10. UTC? Coordinated Universal Time
Backronym: "Universal Time, Coordinated"
Time Standard based on International Atomic Time (TAI)
Not based on solar calculations
UTC replaced GMT as the basis for the main reference time scale or civil time in various regions on 1 January 1972. .

11. ISO 8601
Representation of dates and times is an international standard covering the exchange of date and time-related data.
Avoid misinterpretation of numeric representations of dates and times.
Standardized method of communicating time- based information across time zones by attaching an offset to Coordinated Universal Time (UTC)

12. How ISO 8601 is Done
Date:
Separate Date and Time in UTC:  :01Z
Combined Date and Time in UTC: T07:01Z
Combined Date and Time in MDT: T07:38: :00
Date With Week Number: 2010-W27-3
Ordinal Date:
Duration: P3Y6M4DT12H30M5S
Intervals:
T13:00:00Z/ T15:30:00Z
T13:00:00Z/P1Y2M10DT2H30M
P1Y2M10DT2H30M/ T15:30:00Z
P1Y2M10DT2H30M

13. Modern Calendering/Chronology

14. Julian Reform of the Roman calendar
Was introduced by Julius Caesar in 46 BC
Remained in use into the 20th century in some countries as a civil calendar
The Roman Catholic Church and Protestant churches have replaced the Julian calendar with the Gregorian calendar.
Orthodox Church (with the exception of Estonia and Finland) still use for calculating the dates of moveable feasts.

15. Gregorian Accepted As Civil Calendar Worldwide
Introduced by Pope Gregory XIII
Julian calendar system assumes time between vernal equinoxes is days, when in fact it is about 11 minutes less.
Signed 24 February 1582.

16. Other In-Use Calendars
Buddhist - One era (the current era), and has a year offset by 543 from the current Gregorian year.
Coptic - Based on that used in ancient Egypt. a.k.a. Alexandrian Calendar
In use today by the Coptic Orthodox Church and by farmers tracking the seasons in Egypt.

17. Other In-Use Calendars
Ethiopic – Similar to Coptic in use in Ethiopia
Gregorian-Julian - Switch from one to the other occurs at a configurable date. The default date is , as defined by Pope Gregory XIII.
Islamic - Lunar calendar used in many Muslim countries. Anno Hegiræ

18. Dates, Times, Calendars are Tough

19. The Problem with Java

20. java.util.Date No support for Locale No calendar support
Just a wrapper for # of milliseconds since epoch
Deprecated
Intended to reflect coordinated universal time (UTC), it may not do so exactly, depending on the host environment of the Java Virtual Machine.
A year y is represented by the integer y

21. java.util.Calendar Based on Gregorian Calendar
A month is represented by an integer from 0 to 11; 0 is January, 1 is February, and so forth; thus 11 is December.
Mutable, You can change the object created without other references knowing about it.
Awkward methods
Uses static methods
Calendar.getInstance()
Other Calendars are awkward to extend.

22. Joda Time

23. Joda Time Straight-forward instantiation and methods
ISO 8601 Based, not Gregorian Calendar
Has support for other calendar systems if you need it (Julian, Gregorian-Julian, Coptic, Buddhist)
Includes classes for date times, dates without times, times without dates, intervals and time periods.
Advanced formatting
Well documented and well tested
Immutable!
Months are 1 based
JSR-310

24. Starting off import org.joda.time.DateTime;
DateTime dateTime = new DateTime();
DateTime dt = new DateTime
           (" T23:39: :00");

25. Instant
Moment in the DateTime continuum specified as a number of milliseconds from T00:00Z
Represented by the ReadableInstant interface
Instant, DateTime, MutableDateTime and DateTimeMidnight

26. Interval
Interval of time from one instant to another instant. Both instants are fully specified instants in the DateTime continuum, complete with time zone.
Start instant is inclusive but the end instant is exclusive
Interval and MutableInterval

27. Interval Example
DateTime saleStart = new DateTime (2011, 11, 26, 5, 30, 0, 0);
DateMidnight saleEnd = new DateMidnight(2011, 12, 25);
Interval interval = new Interval(saleStart, saleEnd);
interval.contains(new DateTime()); //false
interval.contains(new DateTime (2011, 12, 12, 20, 12, 44, 332)); //true interval.toDuration().getStandardSeconds(); //Woohoo! sec. sale! Wait what? interval.toPeriod(PeriodType.days()).getDays()); //28 day sale!

28. Duration Duration of time measured in milliseconds
Usually obtained from Interval
Start to Finish
instant + duration = instant

29. Duration Example
DateTime saleStart = new DateTime (2011, 11, 26, 5, 30, 0, 0);
DateMidnight saleEnd = new DateMidnight(2011, 12, 25);
Interval interval = new Interval(saleStart, saleEnd);
interval.contains(new DateTime()); //false
interval.contains(new DateTime (2011, 12, 12, 20, 12, 44, 332)); //true interval.toDuration().getStandardSeconds(); //Woohoo! sec. sale! Wait what? interval.toPeriod(PeriodType.days()).getDays()); //28 day sale!

30. Period instant + period = instant
Period of time defined in terms of fields, for example, 3 years 5 months 2 days and 7 hours
Differs from a duration in that it is inexact in terms of milliseconds
Periods are useful for human interaction
“How many weeks will it take?”

31. Period Side Effects
Given 1 month period, if you add this period to the 1st February (ISO) then you will get the 1st March.
Given 1 month period to the 1st March you will get the 1st April

32. More Period Side Effects
Adding 1 day at the daylight savings boundary. If you use a period to do the addition then either 23 or 25 hours will be added as appropriate.
If you had created a duration equal to 24 hours, then you would end up with the wrong result (maybe)
Period, Mutable Period

33. Period Example
DateTime saleStart = new DateTime (2011, 11, 26, 5, 30, 0, 0);
DateMidnight saleEnd = new DateMidnight(2011, 12, 25);
Interval interval = new Interval(saleStart, saleEnd);
interval.contains(new DateTime()); //false
interval.contains(new DateTime (2011, 12, 12, 20, 12, 44, 332)); //true interval.toDuration().getStandardSeconds(); //Woohoo! sec. sale! Wait what? interval.toPeriod(PeriodType.days()).getDays()); //28 day sale!

34. Partial partial + missing fields + time zone = instant
Partial representation of date / time
A partial does not fully specify a single point in the datetime continuum
Essentially an date or time object to be defined later.
LocalDate, LocalTime, LocalDateTime, YearMonth, MonthDay, Partial

35. Partial Example
LocalDate localDate = new LocalDate(2011, 10, 11); //LocalDate is not time zone or time specific
localDate.minusDays(10).dayOfWeek().getAsText(); //Saturday
LocalTime lunch = new LocalTime(11, 30); //Local Time is not time zone or date specific
DateTime lunchTime = localDate.toDateTime (lunch,DateTimeZone.forID("US/Mountain")); //Bring it together T11:30: :00

36. Chronology Representation of Calendars Default is ISO Chronology
Pluggable
Chronology coptic = CopticChronology.getInstance();

37. Time Zones DateTimeZone zone = DateTimeZone.forID("Europe/London");
DateTimeZone zoneUTC = DateTimeZone.UTC;
DateTimeZone zoneUTC =    DateTimeZone.forOffsetHours(hours);
Based on public TZ

38. Packaging
The public packages are the root package (under org.joda.time) and the format package
The private packages are the base, chrono, convert, field and tz packages.

39. JDK interoperability

40. java.util.Date // from JodaTime to JDK DateTime dt = new DateTime();
Date jdkDate = dt.toDate();
// from JDKTime to Joda
dt = new DateTime(jdkDate);

41. java.util.Calendar // from Joda to JDK DateTime dt = new DateTime();
Calendar jdkCal =    dt.toCalendar(Locale.CHINESE);
// from JDK to Joda
dt = new DateTime(jdkCal);

42. java.util.GregorianCalendar
// from Joda to JDK
DateTime dt = new DateTime();
GregorianCalendar jdkGCal =      dt.toGregorianCalendar();
// from JDK to Joda
dt = new DateTime(jdkGCal);

43. Date and Field Separation
DateTimeField is separate from DateTime to query information.
Done through DateTime.Property
DateTime.Property has then other methods to do get better information

44. Date and Field Example
Given: DateTime dateTime = new DateTime(2011, 7, 12, 20, 0, 0, 0);
DateTime.Property dayOfWeekProperty =     dateTime.dayOfWeek();
dayOfWeekProperty.getAsShortText();        //returns "Mon", "Tue", etc.
dayOfWeekProperty.getAsText();        // returns "Monday",        // "Tuesday", etc.

45. Date and Field Separation (Cont.)
dayOfWeekProperty.getAsText (Locale.FRENCH); // returns "Lundi", etc.
dayOfWeekProperty.get(); // Integer Representation
Other metadata on the minimum and maximum text size, leap status, related durations.

46. “Modifying” DateTimes
Given: DateTime dateTime = new DateTime(2011, 7, 12, 20, 0, 0, 0);
DateTime result =     dt.dayOfWeek().addToCopy(3); // :00:00.000
DateTime result = dateTime.plusDays(3) // :00:00.000
Each provides a copy

47. Mutable DateTime MutableDateTime mdt = dt.toMutableDateTime();
(Intended for creation purposes)
MutableDateTime mdt =     dt.toMutableDateTime();      
// perform various calculations on mdt
DateTime result =     mdt.toDateTime(); //lock it up!

48. Time Zones
// get current moment in default time zone
DateTime dt = new DateTime(); // T13:23: :00
// translate to Eastern time
DateTime dtEastern = dt.withZone (DateTimeZone.forID("US/Eastern")); // T15:23: :00
DateTime dtEastern = dt.withZoneRetainFields (DateTimeZone.forID("US/Eastern")); // T13:23: :00

49. DateTimeFormatter Used to format the DateTime Instant
DateTimeFormatter fmt =    DateTimeFormat.forPattern("yyyyMMdd");
DateTimeFormatter frenchFmt =    fmt.withLocale(Locale.FRENCH);
DateTimeFormatter dateTimeFormatter = ISODateTimeFormat. dateHourMinuteSecond();

50. Direct Format via toString()
DateTime dt = new DateTime();
String a = dt.toString();
String b = dt.toString("dd:MM:yy");
String c = dt.toString ("EEE", Locale.FRENCH);
DateTimeFormatter fmt = ...;
String d = dt.toString(fmt);

51. What's Possible
dateTime.plusDays(20). plusMonths(2).dayOfWeek(). withMaximumValue().plusHours(4). toString ("E MM/dd/yyyy HH:mm:ss.SSS");
Immutability = Automatic Memento

52. Joda-Time Language Libraries
Scala-Time
DateTime.nextMonth < DateTime.now + 2.months
Clj-Time
(hour (date-time ))

53. Testing with JodaTime
// always return the same time // when querying current time
DateTimeUtils.setCurrentMillisFixed(millis) ;
// offset the real time DateTimeUtils.setCurrentMillisOffset (millis);
// reset it back DateTimeUtilssetCurrentMillisSystem()

54. Time Pro Tips
Have a Continuous Integration Server in another time zone
Store Date/Time in Unix Time UTC, Make it your standard
Think of Time Zones as presentation specific
Different computers have different times due to temperature, incorrectness, adjustment, gravity, and more.
Use Joda Time

55. Thank You