How to handle calendar TimeZones using Java?

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.

Thank you all for responding. After a further investigation I got to the right answer. As mentioned by Skip Head, the TimeStamped I was getting from my application was being adjusted to the user's TimeZone. So if the User entered 6:12 PM (EST) I would get 2:12 PM (GMT). What I needed was a way to undo the conversion so that the time entered by the user is the time I sent to the WebServer request. Here's how I accomplished this:

// Get TimeZone of userTimeZone currentTimeZone = sc_.getTimeZone();Calendar currentDt = new GregorianCalendar(currentTimeZone, EN_US_LOCALE);// Get the Offset from GMT taking DST into accountint gmtOffset = currentTimeZone.getOffset(    currentDt.get(Calendar.ERA),     currentDt.get(Calendar.YEAR),     currentDt.get(Calendar.MONTH),     currentDt.get(Calendar.DAY_OF_MONTH),     currentDt.get(Calendar.DAY_OF_WEEK),     currentDt.get(Calendar.MILLISECOND));// convert to hoursgmtOffset = gmtOffset / (60*60*1000);System.out.println("Current User's TimeZone: " + currentTimeZone.getID());System.out.println("Current Offset from GMT (in hrs):" + gmtOffset);// Get TS from User InputTimestamp issuedDate = (Timestamp) getACPValue(inputs_, "issuedDate");System.out.println("TS from ACP: " + issuedDate);// Set TS into CalendarCalendar issueDate = convertTimestampToJavaCalendar(issuedDate);// Adjust for GMT (note the offset negation)issueDate.add(Calendar.HOUR_OF_DAY, -gmtOffset);System.out.println("Calendar Date converted from TS using GMT and US_EN Locale: "    + DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.SHORT)    .format(issueDate.getTime()));

The code's output is: (User entered 5/1/2008 6:12PM (EST)

Current User's TimeZone: EST
Current Offset from GMT (in hrs):-4 (Normally -5, except is DST adjusted)
TS from ACP: 2008-05-01 14:12:00.0
Calendar Date converted from TS using GMT and US_EN Locale: 5/1/08 6:12 PM (GMT)

You say that the date is used in connection with web services, so I assume that is serialized into a string at some point.

If this is the case, you should take a look at the setTimeZone method of the DateFormat class. This dictates which time zone that will be used when printing the time stamp.

A simple example:

SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'");formatter.setTimeZone(TimeZone.getTimeZone("UTC"));Calendar cal = Calendar.getInstance();String timestamp = formatter.format(cal.getTime());