datetime - Java: Adding TimeZone to DateTimeFormatter

Question

The LocalDateTime API gives the possibility to add the TimeZone Name by using the key "z" in the formatter. I get an exception adding this key and don't understand why. I'm looking for something like this example '11:59:22 PM GMT' and not '**... UMT+2**'.

My Code:

public class TimeZone
{
    public static void main(String[] args)
    {
        LocalDateTime now = LocalDateTime.now();
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("hh:mm:ss a z");
        System.out.println(now.format(formatter));
    }
}

The Exception:

Exception in thread "main" java.time.DateTimeException: Unable to extract value: class java.time.LocalDateTime
at java.time.format.DateTimePrintContext.getValue(Unknown Source)
at java.time.format.DateTimeFormatterBuilder$ZoneTextPrinterParser.format(Unknown Source)
at java.time.format.DateTimeFormatterBuilder$CompositePrinterParser.format(Unknown Source)
at java.time.format.DateTimeFormatter.formatTo(Unknown Source)
at java.time.format.DateTimeFormatter.format(Unknown Source)
at java.time.LocalDateTime.format(Unknown Source)
at tz.TimeZone.main(TimeZone.java:12)

Here is the DateTimeFormatter API part:

All letters 'A' to 'Z' and 'a' to 'z' are reserved as pattern letters. The following pattern letters are defined: 
  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   G       era                         text              AD; Anno Domini; A
   u       year                        year              2004; 04
   y       year-of-era                 year              2004; 04
   D       day-of-year                 number            189
   M/L     month-of-year               number/text       7; 07; Jul; July; J
   d       day-of-month                number            10

   Q/q     quarter-of-year             number/text       3; 03; Q3; 3rd quarter
   Y       week-based-year             year              1996; 96
   w       week-of-week-based-year     number            27
   W       week-of-month               number            4
   E       day-of-week                 text              Tue; Tuesday; T
   e/c     localized day-of-week       number/text       2; 02; Tue; Tuesday; T
   F       week-of-month               number            3

   a       am-pm-of-day                text              PM
   h       clock-hour-of-am-pm (1-12)  number            12
   K       hour-of-am-pm (0-11)        number            0
   k       clock-hour-of-am-pm (1-24)  number            0

   H       hour-of-day (0-23)          number            0
   m       minute-of-hour              number            30
   s       second-of-minute            number            55
   S       fraction-of-second          fraction          978
   A       milli-of-day                number            1234
   n       nano-of-second              number            987654321
   N       nano-of-day                 number            1234000000

   V       time-zone ID                zone-id           America/Los_Angeles; Z; -08:30
   z       time-zone name              zone-name         Pacific Standard Time; PST
   O       localized zone-offset       offset-O          GMT+8; GMT+08:00; UTC-08:00;
   X       zone-offset 'Z' for zero    offset-X          Z; -08; -0830; -08:30; -083015; -08:30:15;
   x       zone-offset                 offset-x          +0000; -08; -0830; -08:30; -083015; -08:30:15;
   Z       zone-offset                 offset-Z          +0000; -0800; -08:00;

Can anyone see what the problem is?

Answer 1

LocalDateTime has two fields of type LocalDate and LocalTime.
LocalDate has fields day, month, and year.
LocalTime has fields hour, minute, second, and nano.

Nowhere in that is a time zone given. Which is by nature, since the javadoc of LocalDateTime says:

A date-time without a time-zone

So, if the "local" date/time value is already representing a time in UTC, and you want it formatted saying so, you have multiple options:

• Change the LocalDateTime to a ZonedDateTime by calling atZone():

  System.out.println(time.atZone(ZoneOffset.UTC)
                         .format(DateTimeFormatter.ofPattern("hh:mm:ss a z")));
  

• Specify an override time zone in the formatter by calling withZone():

  System.out.println(time.format(DateTimeFormatter.ofPattern("hh:mm:ss a z")
                                                  .withZone(ZoneOffset.UTC)));
  

• Format with a literal Z character:

  System.out.println(time.format(DateTimeFormatter.ofPattern("hh:mm:ss a 'Z'")));
  

All three of the above outputs:

11:59:22 PM Z

---

Now, if the "local" date/time is really in a different time zone, you can use either of the first two, and just specify the actual zone.

E.g. if time zone is -04:00, use ZoneOffset.ofHours(-4), and you'll get:

11:59:22 PM -04:00

Or if you are in New York, use ZoneId.of("America/New_York"), and you'll get:

11:59:22 PM EDT

If the "local" date/time is for New York, but you want formatted text to be UTC, use both at the same time, i.e.

System.out.println(time.atZone(ZoneId.of("America/New_York"))
                       .format(DateTimeFormatter.ofPattern("hh:mm:ss a z")
                                                .withZone(ZoneOffset.UTC)));

With that, you get the time converted to:

03:59:22 AM Z