如何在UTC 7中将UTC偏移的日期时间值转换为GMT

Question

I have a date time value 2016-12-21T07:48:36 with an offset of UTC+14. How to convert the datetime into equivalent standard GMT time.

I tried with sampleDateFormat.parse() method.But, I am not able to get the TimeZone object for UTC offset like.

sampleDateFormat.setTimeZone(TimeZone.getTimeZone("UTC+14:00"))

Please help me to convert the UTC datetime into standard GMT time in Java 7.

Answer 1

I will assume you have the original date as a string. Do the following:

• Create a SimpleDateFormat and set the timezone to "GMT+14"
• Parse the string value. You get a Date object
• Set the timezone of the SimpleDateFormat to "UTC" (or use a different SimpleDateFormat instance)
• Format the date (if you want the result as a string as well)

Example:

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class ConvertToUTC {
  public static void main(String[] args) throws Exception {

      String dateval = "2016-12-21T07:48:36";
      DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
      df.setTimeZone(TimeZone.getTimeZone("GMT+14"));
      Date date = df.parse(dateval);

      System.out.println(df.format(date)); // GMT+14

      df.setTimeZone(TimeZone.getTimeZone("UTC"));
      System.out.println(df.format(date)); // UTC
  }
}

Answer 2

use "GMT+14:00" instead of "UTC+14:00"

SimpleDateFormat f = new SimpleDateFormat("yyyy-MM-dd'T'hh:mm:ss");
f.setTimeZone(TimeZone.getTimeZone("GMT+14:00"));
final Date d = f.parse("2016-12-21T07:48:36");
f.setTimeZone(TimeZone.getTimeZone("UTC"));
System.out.println(f.format(d)); // -> 2016-12-20T05:48:36

Answer 3

tl;dr

LocalDateTime.parse( "2016-12-21T07:48:36" )        // Parse as a `LocalDateTime` given the lack of an offset or zone. *Not* an actual moment, only a rough approximation of potential moments along a range of about 26-27 hours.
             .atOffset( ZoneOffset.ofHours( 14 ) )  // Assign an offset-from-UTC as context, giving meaning to determine an actual point on the timeline.
             .toInstant()                           // Renders `Instant` object in UTC.

java.time

The modern way is with the java.time classes built into Java 8 and later. Much of the functionality is back-ported to Java 6 & 7 in the ThreeTen-Backport project.

ZoneOffset offset = ZoneOffset.ofHours( 14 ); // fourteen hours ahead of UTC.

Parse the string as a LocalDateTime as it lacks any info about offset or zone. Your input is in standard ISO 8601 format, so no need to specify a formatting pattern.

LocalDateTime ldt = LocalDateTime.parse( "2016-12-21T07:48:36" );

Apply the offset to the local date-time to get an OffsetDateTime an object.

OffsetDateTime odt = ldt.atOffset( offset );

From that, extract an Instant which is always in UTC .

Instant instant = odt.toInstant();

instant.toString(): 2016-12-20T17:48:36Z

In UTC , the value is a different date, the 20th instead of 21st.

See live code at IdeOne.com .

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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 , & SimpleDateFormat .

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

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

Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8 , Java 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 Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP… .

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 .