How to Convert a Timestamp From Any TimeZone to GMT in Java?

How to Handle Calendar TimeZones Using Java

Question:

You have a Timestamp from your application that may originate from any TimeZone. However, the data must be sent to a WebService that assumes it's always in GMT. You need to convert the user's input time from their local TimeZone to GMT without their knowledge.

Answer:

Java's TimeZone handling can be confusing. While Timestamps are typically stored in GMT, the Calendar class uses the system's current TimeZone by default. To address this challenge, consider the following:

import java.util.Calendar;
import java.util.TimeZone;

public class ConvertTimeZone {

    public static void main(String[] args) {
        // Example input time (EST)
        Calendar input = Calendar.getInstance(TimeZone.getTimeZone("EST"));
        input.set(2008, Calendar.MAY, 1, 18, 12, 0);

        // Create a Calendar in GMT
        Calendar gmt = convertToGmt(input);

        // Output the converted time (GMT)
        System.out.println(gmt.getTime()); // 2008-05-01 11:12:00 GMT
    }

    public static Calendar convertToGmt(Calendar cal) {
        Date date = cal.getTime();
        TimeZone tz = cal.getTimeZone();

        long msFromEpochGmt = date.getTime(); // Convert to milliseconds since epoch in GMT
        int offsetFromUTC = tz.getOffset(msFromEpochGmt);

        // Create a new Calendar in GMT and adjust for the offset
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.setTime(date);
        gmtCal.add(Calendar.MILLISECOND, offsetFromUTC);

        return gmtCal;
    }
}

This code takes the following steps:

1. Gets the current time in the user's local TimeZone as a Calendar object.
2. Creates a new Calendar object in GMT.
3. Calculates the offset between the user's local TimeZone and GMT.
4. Adjusts the GMT Calendar by adding the offset to the user's time.
5. The resulting Calendar object now represents the same time in GMT, regardless of the user's TimeZone.

By utilizing this approach, you can accurately convert the user's input time from any TimeZone to GMT, ensuring data compatibility with the WebService.

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