How to make Date locale-independent?

Question

I have a db, that stores dates in OleDateTime format, in GMT timezone. I've implemented a class, extending Date in java to represent that in classic date format. But my class is locale-dependent (I'm in GMT+2). Therefore, it converts the date in the db as date - 2 hours. How do I make it convert the date correctly? I want my class to be locale-independent, always using GMT timezone. Actually, the question is:

class MyOleDateTime extends Date {

    static {
        Locale.setDefault(WhatGoesHere?)
    }

    // ... some constructors
    // ... some methods
}

Answer 1

Well, it's better to use the Calendar object like suggested in other answers. However, if you really want to set global timezone, you can use TimeZone.setDefault(TimeZone.getTimeZone("UTC")); early in your application code. There is also user.timezone Java system property.

Also (just fun to know), it appears that the only country actually living by GMT/UTC time (without daylight saving changes) is Liberia.

In fact, Date objects per se are always locale- and timezone-independent. Its getTime() method will always return the number of milliseconds passed since January 1, 1970 00:00:00 (not counting leap seconds) in UTC. But if you want to get something else than milliseconds, you have to use Calendar, which is timezone-dependent. But it is the right way to go. You don't use that deprecated methods in Date class, do you?

Answer 2

As Michael Borgwardt has already said, the Java Date object does not know anything about timezones. It's just a wrapper for a number of milliseconds since 01-01-1970 00:00:00 UTC.

You start dealing with timezones only when you for example convert the Date object to a String using a DateFormat. You set the timezone on the DateFormat to specify in which timezone you want to see the Date.

DateFormat df = new SimpleDateFormat("dd-MM-yyyy HH:mm:ss Z");
df.setTimeZone(TimeZone.getTimeZone("UTC"));

String text = df.format(date);  // text will contain date represented in UTC

Answer 3

A Date is locale-independent, always using GMT timezone. It's just a wrapper around a millisecond timestamp in GMT (more correctly: UTC).

The only things in Date that are timezone dependant are the deprecated methods like getDay() - that's why they're deprecated. Those use the default time zone. The correct thing to do is to avoid using those deprecated methods - not to set the default timezone to UTC! That could cause problems elsewhere, and you can't prevent other parts of the code from setting the default timezone to something else.

Answer 4

Use a Calendar object:

Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("GMT"),
                                    locale);

Answer 5

Here's a snippet I used to calculate the GMT offset from the Calendar instance and format it. I appreciate all the help I've gotten from this site, its nice to contribute. I hope this helps someone somewhere. Enjoy.

Calendar calInst = Calendar.getInstance();

//calculate the offset to keep calendar instance GMT
int gmtOffsetMilli = calInst.get(Calendar.ZONE_OFFSET);
long gmtOffsetHr = TimeUnit.HOURS.convert(gmtOffsetMilli, TimeUnit.MILLISECONDS);

calInst = Calendar.getInstance(TimeZone.getTimeZone("GMT " + gmtOffsetHr));