如何设置java.util.Date的偏移量？

Question

I have a Date as a String - 15Sep20162040 , which I have to format it into another format with Timezone as 2016-09-15T20:40:00+0400 .

What I did to do it as follows:

import java.text.ParseException;
import java.text.SimpleDateFormat;

public class DateFormatExample {

    private static SimpleDateFormat offsetDateFormat = new SimpleDateFormat(
            "yyyy-MM-dd'T'HH:mm:ssZ");

    private static SimpleDateFormat dateFormatter = new SimpleDateFormat(
            "ddMMMyyyyHHmm");

    public static void main(String[] args) throws ParseException {
        String date = "15Sep20162040";
        String result = offsetDateFormat.format(dateFormatter.parse(date));
        System.out.println(result); // 2016-09-15T20:40:00+0400         

    }
}

Now, I have to modify the output based on timezone difference, for example if difference is +0100 , output should resemble as: 2016-09-15T20:40:00+0100 and if difference is -0200 , output should resemble as: 2016-09-15T20:40:00-0200 .

How can I achieve it?

Answer 1

You can use SimpleDateFormat 's setTimeZone method as below:

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.TimeZone;

public class DateFormatExample {

    private static SimpleDateFormat offsetDateFormat = new SimpleDateFormat(
            "yyyy-MM-dd'T'HH:mm:ssZ");

    private static SimpleDateFormat dateFormatter = new SimpleDateFormat(
            "ddMMMyyyyHHmm");

    public static void main(String[] args) throws ParseException {
        String date = "15Sep20162040";
        String result = offsetDateFormat.format(dateFormatter.parse(date));
        System.out.println(result); // 2016-09-15T20:40:00+0400
        offsetDateFormat.setTimeZone(TimeZone.getTimeZone("GMT-8:00"));
        result = offsetDateFormat.format(dateFormatter.parse(date));
        System.out.println(result);
    }
}

If you simply want to change the timezone at the end of result , please try the following:

    String offset = "GMT-8:00";
    String date = "15Sep20162040";
    date = date+" "+offset;
    SimpleDateFormat dateFormatter2 = new SimpleDateFormat("ddMMMyyyyHHmm Z");
    SimpleDateFormat offsetDateFormat2 = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ");
    offsetDateFormat2.setTimeZone(TimeZone.getTimeZone(offset));
    String result = offsetDateFormat2.format(dateFormatter2.parse(date));
    System.out.println(result);

Hope this helps.

Answer 2

tl;dr

ZonedDateTime zdt = 
    LocalDateTime.parse ( "15Sep20162040" , 
                          DateTimeFormatter.ofPattern ( "ddMMMyyyyHHmm" )
                                           .withLocale( Locale.English ) 
                        )
                 .atZone ( ZoneId.of ( "America/Puerto_Rico" ) );

2016-09-15T20:40-04:00[America/Puerto_Rico]

zdt.atZone( ZoneId.of ( "Pacific/Auckland" ) )  // Same moment viewed through different wall-clock time

2016-09-16T12:40+12:00[Pacific/Auckland]

Using java.time

Avoid the troublesome old date-time classes, now supplanted by the java.time classes.

Define a formatting pattern to match your input string.

String input = "15Sep20162040";
DateTimeFormatter f = DateTimeFormatter.ofPattern ( "ddMMMyyyyHHmm" ).withLocale ( Locale.ENGLISH );

By the way, this is a terrible format for a date-time string. It assumes English, abuses English with incorrect abbreviation of month name, and is confusing and ambiguous. Instead, use the standard ISO 8601 formats when serializing date-time values to text.

Un-zoned

Parse the input string as a LocalDateTime since it lacks any info about offset-from-UTC or time zone.

LocalDateTime ldt = LocalDateTime.parse ( input , f );

Understand that without an offset or time zone, this LocalDateTime object has no real meaning. It represents many possible moments, but not a specific point on the timeline. For example, noon in Auckland NZ is a different moment than noon in Kolkata India which is an earlier moment than noon in Paris France.

Assign an offset-from-UTC

You indicate this date-time was intended to be a moment with an offset-from-UTC of four hours behind UTC (-04:00). So next we apply a ZoneOffset to get a OffsetDateTime object.

Tip: Always include the colon and the minutes and padding zeros in your offset-from-UTC strings. While not required by the ISO 8601 standard, common software libraries and protocols expect the fuller formatting.

ZoneOffset offset = ZoneOffset.ofHours( -4 ); 
OffsetDateTime odt = ldt.atOffset( offset );

Assign a time zone

If by your context you knew of a time zone rather than a mere offset, use a ZoneId to instantiate a ZonedDateTime object. A time zone is an offset plus a set of rules for handling anomalies such as Daylight Saving Time (DST) .

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( "America/Puerto_Rico" );
ZonedDateTime zdt = ldt.atZone( z );

Different time zones

Your question is not clear near the end, about changing offsets. If your goal is to view the date-time through the various lenses of various time zones, you can easily adjust by creating new ZonedDateTime objects. Assign a different time zone to each.

Note that all these date-time objects (zdt, zKolkata, and zAuckland) represent the same moment, the same point on the timeline. Each presents a different wall-clock time but for the same simultaneous moment .

ZoneId zKolkata = ZoneId.of ( "Asia/Kolkata" );
ZonedDateTime zdtKolkata = zdt.withZoneSameInstant ( zKolkata );

ZoneId zAuckland = ZoneId.of ( "Pacific/Auckland" );
ZonedDateTime zdtAuckland = zdt.withZoneSameInstant ( zAuckland );

System.out.println ( "input: " + input + " | ldt: " + ldt + " | odt: " + odt + " | zdt: " + zdt + " | zdtKolkata " + zdtKolkata + " | zdtAuckland: " + zdtAuckland );

Dump to console.

input: 15Sep20162040 | ldt: 2016-09-15T20:40 | odt: 2016-09-15T20:40-04:00 | zdt: 2016-09-15T20:40-04:00[America/Puerto_Rico] | zdtKolkata 2016-09-16T06:10+05:30[Asia/Kolkata] | zdtAuckland: 2016-09-16T12:40+12:00[Pacific/Auckland]

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old 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.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP (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.