Introduction to commonly used time APIs and usage methods in Java

1. The Clock class can be used to access the current date and time. Clock can get the current time zone instead of System.currenttimeMillis().

Clock clock = Clock.systemDefaultZone();long millis = clock.millis();
 
Instant instant = clock.instant();
Date legacyDate = Date.from(instant);   // legacy java.util.Date

2. Time is represented by zoneId, and zoneId can be accessed through the static factory.

System.out.println(ZoneId.getAvailableZoneIds());// prints all available timezone ids
 
ZoneId zone1 = ZoneId.of("Europe/Berlin");
ZoneId zone2 = ZoneId.of("Brazil/East");
System.out.println(zone1.getRules());
System.out.println(zone2.getRules());
 
// ZoneRules[currentStandardOffset=+01:00]
// ZoneRules[currentStandardOffset=-03:00]

3. LocalTime means there is no time zone, such as 10pm or 17:30:15.

LocalTime now1 = LocalTime.now(zone1);
LocalTime now2 = LocalTime.now(zone2);
 
System.out.println(now1.isBefore(now2));  // false
 
long hoursBetween = ChronoUnit.HOURS.between(now1, now2);
long minutesBetween = ChronoUnit.MINUTES.between(now1, now2);
 
System.out.println(hoursBetween);       // -3
System.out.println(minutesBetween);     // -239

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