Trail: Date Time
Lesson: Standard Calendar
Overview
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Overview

There are two basic ways to represent time. One way represents time in human terms, referred to as human time, such as year, month, day, hour, minute and second. The other way, machine time, measures time continuously along a timeline from an origin, called the epoch, in nanosecond resolution. The Date-Time package provides a rich array of classes for representing date and time. Some classes in the Date-Time API are intended to represent machine time, and others are more suited to representing human time.

First determine what aspects of date and time you require, and then select the class, or classes, that fulfill those needs. When choosing a temporal-based class, you first decide whether you need to represent human time or machine time. You then identify what aspects of time you need to represent. Do you need a time zone? Date and time? Date only? If you need a date, do you need month, day, and year, or a subset?


Terminology: The classes in the Date-Time API that capture and work with date or time values, such as Instant, LocalDateTime, and ZonedDateTime, are referred to as temporal-based classes (or types) throughout this tutorial. Supporting types, such as the TemporalAdjuster interface or the DayOfWeek enum, are not included in this definition.

For example, you might use a LocalDate object to represent a birth date, because most people observe their birthday on the same day, whether they are in their birth city or across the globe on the other side of the international date line. If you are tracking astrological time, then you might want to use a LocalDateTime object to represent the date and time of birth, or a ZonedDateTime, which also includes the time zone. If you are creating a time-stamp, then you will most likely want to use an Instant, which allows you to compare one instantaneous point on the timeline to another.

The following table summarizes the temporal-based classes in the java.time package that store date and/or time information, or that can be used to measure an amount of time. A check mark in a column indicates that the class uses that particular type of data and the toString Output column shows an instance printed using the toString method. The Where Discussed column links you to the relevant page in the tutorial.

Class or Enum Year Month Day Hours Minutes Seconds* Zone Offset Zone ID toString Output Where Discussed
Instant          
checked
    2013-08-20T15:16:26.355Z Instant Class
LocalDate
checked
checked
checked
          2013-08-20 Date Classes
LocalDateTime
checked
checked
checked
checked
checked
checked
    2013-08-20T08:16:26.937 Date and Time Classes
ZonedDateTime
checked
checked
checked
checked
checked
checked
checked
checked
2013-08-21T00:16:26.941+09:00[Asia/Tokyo] Time Zone and Offset Classes
LocalTime      
checked
checked
checked
    08:16:26.943 Date and Time Classes
MonthDay  
checked
checked
          --08-20 Date Classes
Year
checked
              2013 Date Classes
YearMonth
checked
checked
            2013-08 Date Classes
Month  
checked
            AUGUST DayOfWeek and Month Enums
OffsetDateTime
checked
checked
checked
checked
checked
checked
checked
  2013-08-20T08:16:26.954-07:00 Time Zone and Offset Classes
OffsetTime      
checked
checked
checked
checked
  08:16:26.957-07:00 Time Zone and Offset Classes
Duration     ** ** **
checked
    PT20H (20 hours) Period and Duration
Period
checked
checked
checked
      *** *** P10D (10 days) Period and Duration
* Seconds are captured to nanosecond precision.
** This class does not store this information, but has methods to provide time in these units.
*** When a Period is added to a ZonedDateTime, daylight saving time or other local time differences are observed.

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