Dates and Times - Beginning Java 8 Fundamentals - page 427

Representing the Units of Datetime Fields
Time is measured in units such as years, months, days, hours, minutes, seconds, weeks, etc. An instance of the
TemporalUnit interface in the java.time.temporal package represents a unit of time. The ChronoUnit in the same
package contains the following constants to represent units of time: CENTURIES , DAYS , DECADES , ERAS , FOREVER ,
HALF_DAYS , HOURS , MICROS , MILLENNIA , MILLIS , MINUTES , MONTHS , NANOS , SECONDS , WEEKS , and YEARS .
The ChronoUnit enum implements the TemporalUnit interface. Therefore, all constants in the enum are an
instance of the TemporalUnit .

■ Constants for some datetime units that are specific to the isO-8601 calendar system are declared in the
IsoFields class. For example, IsoFields.QUARTER_YEARS and IsoFields.WEEK_BASED_YEARS represent isO-8601
based a quarter-year (3 months) and a week-based year (52 or 53 weeks), respectively. the isO-8601 standard considers
a seven day period as a week; a week starts on a monday; the first calendar week of the year is the one that includes the
first thursday of the year; the first week of a year may start in the previous year and the last week of a year may end in
the succeeding year. this may result in a 53 weeks in a year. For example, the first week of 2009 started on
December 29, 2008 and the last week on December 29, 2009, making 2009 a 53 week year.

Tip

Datetime classes provide two methods, minus() and plus() . They take an amount of time and the unit of time
to return a new datetime by subtracting and adding the specified time. Convenience methods such as minusDays() ,
minusHours() , plusDays() , plusHours() , etc. are also provided by the applicable classes to subtract and add time.
The following snippet of code illustrates the use of the ChronoUnit enum constants with these methods:

import java.time.LocalDateTime;
import java.time.temporal.ChronoUnit;
...
LocalDateTime now = LocalDateTime.now();
// Get the date time 4 days ago
LocalDateTime ldt2 = now.minus(4, ChronoUnit.DAYS);
// Use the minusDays() method to get the same result
LocalDateTime ldt3 = now.minusDays(4);
// Get date and time 4 hours later
LocalDateTime ldt4 = now.plus(4, ChronoUnit.HOURS);

// Use the plusHours() method to get the same result
LocalDateTime ldt5 = now.plusHours(4);

System.out.println("Current Datetime: " + now);
System.out.println("4 days ago: " + ldt2);
System.out.println("4 days ago: " + ldt3);
System.out.println("4 hours after: " + ldt4);
System.out.println("4 hours after: " + ldt5);