How to Convert a Timestamp from Local Time Zone to GMT in Java?

How to Handle Time Zones in Timestamp Calculations in Java

When working with timestamps in Java applications, it is important to consider time zones to ensure accurate calculations and data handling. In this particular scenario, a user may enter a timestamp in their local time zone (EST), but the service expects it to be in GMT.

To convert the timestamp from the user's local time zone to GMT, we need to account for the difference in time offsets. Here's an example of how to overcome this challenge using Java's Calendar class:

public static Calendar convertToGmt(Calendar cal) {

    Date date = cal.getTime();
    TimeZone tz = cal.getTimeZone();
    log.debug("input calendar has date [" + date + "]");

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

    // Get offset from UTC in milliseconds
    int offsetFromUTC = tz.getOffset(msFromEpochGmt);
    log.debug("offset is " + offsetFromUTC);

    // Create GMT calendar and adjust date based on offset
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.setTime(date);
    gmtCal.add(Calendar.MILLISECOND, offsetFromUTC);
    log.debug("Created GMT cal with date [" + gmtCal.getTime() + "]");

    return gmtCal;
}

By utilizing this method, you can convert the timestamp to GMT, ensuring it matches the service's expectations, regardless of the user's location or time zone settings.

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