How to make DateFormat to parse String date to the assigned timezone

Question

I have a string date "20160104" of "yyyyMMdd" and it is of GMT-12 timezone.

Have been trying to parse the same to a Date object in Java. The expected result is

FastTime = 1451908800000. Monday, January 4, 2016 12:00:00 PM GMT

Tried something like this

DateFormat gmtMinus12 = new SimpleDateFormat("yyyyMMdd");
gmtMinus12.setTimeZone(TimeZone.getTimeZone("Etc/GMT-12"));
// gmtMinus12.getCalendar().setTimeZone(TimeZone.getTimeZone("Etc/GMT-12"));
gmtMinus12
    .setCalendar(Calendar.getInstance(TimeZone.getTimeZone("Etc/GMT-12")));

try {
  Date date = gmtMinus12.parse("20160104");

} catch (ParseException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
}

But the parsed output is

FastTime = 1451822400000 . Sunday, January 3, 2016 12:00:00 PM GMT

How do I make the DateFormat to assume the same timezone with which it is assigned.

Any help is greatly appreciated.

Thanks in advance.

Answer 1

java.time

You are using troublesome old date-time classes now supplanted by the java.time classes.

The LocalDate class represents a date-only value, without time-of-day and without time zone.

DateTimeFormatter f = DateTimeFormatter.ofPattern ( "uuuuMMdd" );
LocalDate ld = LocalDate.parse ( "20160104" , f );

Now assign a time-of-day and an offset-from-UTC to deliver a OffsetDateTime object.

ZoneOffset offset = ZoneOffset.ofHours ( -12 );
LocalTime lt = LocalTime.MIN;  // 00:00
OffsetTime ot = OffsetTime.of ( lt , offset );
OffsetDateTime odt = ot.atDate ( ld );

odt.toString(): 2016-01-04T00:00-12:00

To see that same moment in UTC, extract an Instant.

Instant instant = odt.toInstant();

instant.toString(): 2016-01-04T12:00:00Z

As you expected, the time-of-day is noon on the 4th in UTC.

Time zone

You should know that GMT-12 is not a time zone. It is an offset-from-UTC.

A time zone is an offset plus a set of rules for handling anomalies such as Daylight Saving Time (DST).

If you know the intended time zone, you should apply it rather than a mere offset. Specify a proper time zone name in the format of continent/region. Never use the 3-4 letter abbreviation such as EST or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "Asia/Kamchatka" );
ZonedDateTime zdt = odt.atZoneSameInstant( z );

zdt.toString(): 2016-01-05T00:00+12:00[Asia/Kamchatka]

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, .Calendar, & java.text.SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to java.time.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

• Java SE 8 and SE 9 and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • See How to use….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Answer 2

Your real problem is not just using an old and awkward API but using the wrong offset. Please use

Etc/GMT+12 (=UTC-12:00) instead of Etc/GMT-12 (=UTC+12:00).

The outdated Etc/...-notation is very confusing because it uses the opposite sign of standard timezone offsets, see also the embbedded translation table in TZ-database.