Date and Time in Java 8

  • How to get current date and/or time in Java ? 
LocalDate.now()  //Output: 2021-06-20
LocalTime.now()  //Output: 23:22:17.939243900
LocalDateTime.now()  //Output: 2021-06-20T23:13:37.193359900
ZonedDateTime.now()  //Output: 2021-06-20T23:14:21.295180600-04:00[America/New_York]
  • How to convert a String to date and time in Java ? 
public static LocalDateTime StringToDate(){
    String date = "01-October-2021 23:59:00.123";
    DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("dd-MMMM-yyyy HH:mm:ss.SSS");
    LocalDateTime localDateTime = LocalDateTime.parse(date, dateTimeFormatter);
    return localDateTime;
}

For date strings with ‘T’ if you face the below exception: 

Exception in thread "main" java.time.format.DateTimeParseException: Text '2021-09-30T23:50:36.4259546' could not be parsed at index 10

add ‘T’ as shown in the below format: 

  public static void main(String args[]){
        String now = "2021-09-30T23:50:36.4259546";
        System.out.println(" " + LocalDateTime.parse(now, 
                DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSSSSS")));
    }
//OUTPUT: 2021-09-30T23:50:36.425954600
  • How to create a Date Time instance with a previous date time value ?

Either you can parse the String as shown above. Or use the “of” factory method. 

//of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanoOfSecond)

LocalDateTime.of(2021,10,31,23,59,59,283948576); 

LocalDate.of(2021,10,31);
  • How to specify milliseconds / microseconds / nanoseconds in DateTimeFormatter in Java ?
 Formatter SymbolFormatted examples.
Day of monthd/dd2/02
MonthM/MM/MMM/MMMM6/06/Jun/June
Yearyy/yyyy21/2021
Hour in day(0-23)H/HH2/02 , 14/14
Hour in am/pm (1-12)h/hh4/04
Minuesm/mm9/09
Secondsss50
MillisecondsSSS922
MicrosecondsSSSSSS922600
Nanosecondsn922600200
TimeZonez / zzzz / VVEDT / Eastern Daylight Time /America/New_York
OffsetZ-0400
EraG/GGGGAD/ Anno Domini
DateTimeFormatter symbols / pattern.

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/time/format/DateTimeFormatter.html

  • Are Java8 Date/Time APIs thread safe ?

Yes. They are thread safe and that is one of main advantages of using Java8 Date/Time APIs against the APIs of previous java versions (Java7 and below).

  • How to print date/time in a specific format ? 
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("dd-MMMM-yyyy HH:mm:ss.n zzzz VV GGGG");
System.out.println(ZonedDateTime.now().format(dateTimeFormatter));

//Output: 20-June-2021 23:42:43.646265900 Eastern Daylight Time America/New_York Anno Domini
  • How to convert a Date Time to a different Time Zone ?

ZonedDateTime preserves the Time Zone Data. If you want to change the timezone to a different time zone, use “withZoneSameInstant” api. 

System.setProperty("user.timezone", "UTC");
ZonedDateTime zonedDateTimeUTC = ZonedDateTime.now();

System.out.println(zonedDateTimeUTC);    //Output: 2021-06-27T03:44:06.031687100Z[UTC]

ZonedDateTime zonedDateTimeInEastern = zonedDateTimeUTC.withZoneSameInstant(ZoneId.of("US/Eastern"));

System.out.println(zonedDateTimeInEastern); //Output: 2021-06-26T23:44:06.031687100-04:00[US/Eastern]

List of all ZoneIds in java can be found here.

  • How to get epoch milliseconds from LocalDateTime ? And how to convert milliseconds to LocalDateTime ? 
public static void main(String args[]){
    LocalDateTime ldt = LocalDateTime.of(2021, 8, 23, 11, 14, 10);
    System.out.println(toMilliSeconds(ldt));
    System.out.println(toLocalDateTime(1629731650000l));
}

public static long toMilliSeconds(LocalDateTime ldt){
    return ldt.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli();
}

public static LocalDateTime toLocalDateTime(long milliseconds){
    return Instant.ofEpochMilli(milliseconds).atZone(ZoneId.systemDefault()).toLocalDateTime();
}

// Output: 
1629731650000
2021-08-23T11:14:10

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