Java Date Time conversion to given timezone

问题

I have a DateTime in the format of Tue, 30 Apr 2019 16:00:00 +0800 which is RFC 2822 formatted date

I need to convert this to the given timezone in the DateTime which is +0800

So if i summarized,

DateGiven = Tue, 30 Apr 2019 16:00:00 +0800
DateWanted = 01-05-2019 00:00:00

How can i achieve this in Java? I have tried the below code but it gives 08 hours lesser than the current time which is

30-04-2019 08:00:00

Code i tried

String pattern = "EEE, dd MMM yyyy HH:mm:ss Z";
SimpleDateFormat format = new SimpleDateFormat(pattern);
Date startDate = format.parse(programmeDetails.get("startdate").toString());

//Local time zone   
 SimpleDateFormat dateFormatLocal = new SimpleDateFormat("yyyy-MMM-dd HH:mm:ss");

//Time in GMT
Date dttt= dateFormatLocal.parse( dateFormatGmt.format(startDate) );

回答1:

You are on right approach but just use java-8 date time API module, first create DateTimeFormatter with the input format representation

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss Z");

And then use OffsetDateTime to parse string with offset

OffsetDateTime dateTime = OffsetDateTime.parse("Tue, 30 Apr 2019 16:00:00 +0800",formatter);

And the call the toLocalDateTime() method to get the local time

LocalDateTime localDateTime = dateTime.toLocalDateTime();  //2019-04-30T16:00

If you want the output in particular format again you can use DateTimeFormatter

localDateTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME)   //2019-04-30T16:00:00

Note : As @Ole V.V pointed in comment, after parsing the input string into util.Date you are getting the UTC time

The class Date represents a specific instant in time, with millisecond precision.

So now if you convert the parsed date time into UTC you get 2019-04-30T08:00Z without offset, so you can use withOffsetSameInstant to convert it into any particular timezone

dateTime.withOffsetSameInstant(ZoneOffset.UTC)

回答2:

You misunderstood. According to RFC 2822 +0800 means that an offset of 8 hours 0 minutes has already been applied to the time compared to UTC. So the output you got was the correct GMT time.

java.time

I recommend you skip the old and outdated classes SimpleDateFOrmat and Date. It’s much nicer to work with java.time, the modern Java date and time API. Furthermore it has the RFC format built in, so we don’t need to write our own formatter.

    OffsetDateTime parsedDateTime = OffsetDateTime
            .parse("Tue, 30 Apr 2019 16:00:00 +0800",
                    DateTimeFormatter.RFC_1123_DATE_TIME);

    ZonedDateTime dateTimeInSingapore
            = parsedDateTime.atZoneSameInstant(ZoneId.of("Asia/Singapore"));
    System.out.println("In Singapore: " + dateTimeInSingapore);

    OffsetDateTime dateTimeInGmt
            = parsedDateTime.withOffsetSameInstant(ZoneOffset.UTC);
    System.out.println("In GMT:       " + dateTimeInGmt);

Output:

In Singapore: 2019-04-30T16:00+08:00[Asia/Singapore]
In GMT:       2019-04-30T08:00Z

The built-in formatter is named RFC_1123_DATE_TIME because the same format is used in multiple Requests for Comments (RFCs).

Links

• RFC 2822 Internet Message Format
• Oracle tutorial: Date Time explaining how to use java.time.

回答3:

with the help of @ole v.v's explanation i have separated the datetime value for two 1. time 2. timezone

then i used this coding to extract the datetime which is related to the given timezone

//convert datetime to give timezone 
    private static String DateTimeConverter (String timeVal, String timeZone)
    {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");


    SimpleDateFormat offsetDateFormat2 = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ");

        offsetDateFormat2.setTimeZone(TimeZone.getTimeZone(timeZone));
        String result =null;
        try {
            result = offsetDateFormat2.format(format.parse(timeVal));
        } catch (java.text.ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return result;
    }

来源：https://stackoverflow.com/questions/58830202/java-date-time-conversion-to-given-timezone