How to handle calendar TimeZones using Java?

by 
public static Calendar convertToGmt(Calendar cal) {

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

    log.debug("input calendar has date [" + date + "]");

    //Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT 
    long msFromEpochGmt = date.getTime();

    //gives you the current offset in ms from GMT at the current date
    int offsetFromUTC = tz.getOffset(msFromEpochGmt);
    log.debug("offset is " + offsetFromUTC);

    //create a new calendar in GMT timezone, set to this date and add the 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;
}

Here’s the output if I pass the current time (“12:09:05 EDT” from Calendar.getInstance()) in:

DEBUG – input calendar has date [Thu Oct 23 12:09:05 EDT 2008]
DEBUG – offset is -14400000
DEBUG – Created GMT cal with date [Thu Oct 23 08:09:05 EDT 2008]

12:09:05 GMT is 8:09:05 EDT.

The confusing part here is that Calendar.getTime() returns you a Date in your current timezone, and also that there is no method to modify the timezone of a calendar and have the underlying date rolled also. Depending on what type of parameter your web service takes, your may just want to have the WS deal in terms of milliseconds from epoch.

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