Java: Date Time

• Prefer java.time package introduce in Java 8
  • Prefer java.time.format.DateTimeFormatter than java.text.SimpleDateFormatter
  • Prefer java.time.Instant than java.sql.Timestamp

The time since January 1st 1970 in milliseconds

• System.currentTimeMillis()
• new Date().getTime()
• Instant.now().toEpochMilli()
• new Timestamp(System.currentTimeMillis()).getTime()

jshell> System.currentTimeMillis()
==> 1608325829399

jshell> new Date().getTime() == System.currentTimeMillis()
==> true

jshell> import java.time.Instant

jshell> Instant.now()
==> 2020-12-18T21:12:26.327318Z

jshell> Instant.now().toEpochMilli() == System.currentTimeMillis()
==> true

jshell> import java.sql.Timestamp

jshell> Timestamp.from(Instant.now())
==> 2020-12-18 13:15:36.259147

jshell> Instant now = Instant.now()
==> 2020-12-18T21:16:51.373129Z

jshell> Timestamp.from(now).getTime() == now.toEpochMilli()
==> true

Measure Code Execution Duration

long startTime = System.nanoTime(); 
// ... the code being measured 
long endTime = System.nanoTime(); 
long elapsed = endTime - startTime; 

Instant start = Instant.now(); 
// ... the code being measured 
Instant end = Instant.now(); 
Duration duration = Duration.between(start, end);

import java.util.concurrent.TimeUnit;

public class Timer {
  private long start;
  private long end;

  public Timer() {
  }

  public static Timer startTimer() {
    Timer t = new Timer();
    t.start();
    return t;
  }

  public void start() {
    this.start = System.nanoTime();
  }

  public void end() {
    this.end = System.nanoTime();
  }

  public long getElapsedNano() {
    if (end == 0) {
      end();
    }
    return end - start;
  }

  public long getElapsed(TimeUnit unit) {
    return unit.convert(getElapsedNano(), TimeUnit.NANOSECONDS);
  }
}

java.util.TimeZone

jshell> TimeZone.getAvailableIDs()
 ==> String[629] { "Africa/Abidjan", "Africa/Accra", "Africa/Addis_Ababa", "Africa/Algiers", "Africa/Asmara", "Africa/Asmera", "Africa/Bamako", "Africa/Bangui", "Africa/Banjul", "Africa/Bissau", "Africa/Blantyre", "Africa/Brazzaville", "Africa/Bujumbura", "Africa/Cairo", "Africa/Casablanca", "Africa/Ceuta", "Africa/Conakry", "Africa/Dakar", "Africa/Dar_es_Salaam", "Africa/Djibouti", "Africa/Douala", "Africa/El_Aaiun", "Africa/Freetown", "Africa/Gaborone", "Africa/Harare", "Africa/Johannesburg", "Africa/Juba", "Africa/Kampala", "Africa/Khartoum", "Africa/Kigali", "Africa/Kinshasa", "Africa/Lagos", "Africa/Libreville", "Africa/Lome", "Africa/Luanda", "Africa/Lu … US/Indiana-Starke", "US/Michigan", "US/Mountain", "US/Pacific", "US/Pacific-New", "US/Samoa", "UTC", "Universal", "W-SU", "WET", "Zulu", "EST", "HST", "MST", "ACT", "AET", "AGT", "ART", "AST", "BET", "BST", "CAT", "CNT", "CST", "CTT", "EAT", "ECT", "IET", "IST", "JST", "MIT", "NET", "NST", "PLT", "PNT", "PRT", "PST", "SST", "VST" }

java.time.format.DateTimeFormatter

jshell> import java.time.format.DateTimeFormatter
jshell> import java.time.LocalDate

jshell> DateTimeFormatter formatter = DateTimeFormatter.BASIC_ISO_DATE;
jshell> formatter.format(LocalDate.now())
==> "20201218"

jshell> DateTimeFormatter.ISO_DATE_TIME.format(LocalDateTime.now())
==> "2020-12-18T13:54:34.593405"

jshell> DateTimeFormatter.ISO_TIME.format(LocalTime.now())
==> "13:55:18.595585"

jshell> DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").format(LocalDateTime.now())
==> "2020-12-18 14:01:11"

java.time Package

java.time – The main API for dates, times, instants, and durations

Clock	A clock providing access to the current instant, date and time using a time-zone.
Duration	A time-based amount of time, such as ‘34.5 seconds’.
Instant	An instantaneous point on the time-line.
LocalDate	A date without a time-zone in the ISO-8601 calendar system, such as 2007-12-03.
LocalDateTime	A date-time without a time-zone in the ISO-8601 calendar system, such as 2007-12-03T10:15:30.
LocalTime	A time without a time-zone in the ISO-8601 calendar system, such as 10:15:30.
MonthDay	A month-day in the ISO-8601 calendar system, such as --12-03.
OffsetDateTime	A date-time with an offset from UTC/Greenwich in the ISO-8601 calendar system, such as 2007-12-03T10:15:30+01:00.
OffsetTime	A time with an offset from UTC/Greenwich in the ISO-8601 calendar system, such as 10:15:30+01:00.
Period	A date-based amount of time in the ISO-8601 calendar system, such as ‘2 years, 3 months and 4 days’.
Year	A year in the ISO-8601 calendar system, such as 2007.
YearMonth	A year-month in the ISO-8601 calendar system, such as 2007-12.
ZonedDateTime	A date-time with a time-zone in the ISO-8601 calendar system, such as 2007-12-03T10:15:30+01:00 Europe/Paris.
ZoneId	A time-zone ID, such as Europe/Paris.
ZoneOffset	A time-zone offset from Greenwich/UTC, such as +02:00.