How to Convert Java Timestamps to Different Time Zones?

Converting Java Timestamps to Different Time Zones

In Java, timestamps are initially in Greenwich Mean Time (GMT) by default. However, when dealing with user inputs from various time zones, converting timestamps to a user's specific time zone becomes necessary.

Consider this scenario: a user in Eastern Standard Time (EST) provides a timestamp of "5/1/2008 6:12 PM (EST)". The web service expects the timestamp in GMT, which is "5/1/2008 6:12 PM (GMT)".

To accurately convert timestamps across different time zones, a custom converter is needed. Here's the code for convertToGmt():

public static Calendar convertToGmt(Calendar cal) {

    Date date = cal.getTime();
    TimeZone tz = cal.getTimeZone();

    // Milliseconds since epoch GMT
    long msFromEpochGmt = date.getTime();

    // Current offset from GMT at current date
    int offsetFromUTC = tz.getOffset(msFromEpochGmt);

    // Create a GMT calendar, set time, and add offset
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.setTime(date);
    gmtCal.add(Calendar.MILLISECOND, offsetFromUTC);

    return gmtCal;
}

This method converts the input calendar to a GMT calendar, ensuring the timestamp is accurate in GMT for the web service. Note that this method does not currently handle daylight saving time changes.

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