Converting local timestamp to UTC timestamp in Java

Question

I have a milliseconds-since-local-epoch timestamp that I'd like to convert into a milliseconds-since-UTC-epoch timestamp. From a quick glance through the docs it looks like something like this would work:

int offset = TimeZone.getDefault().getRawOffset();
long newTime = oldTime - offset;

Is there a better way to do this?

Answer 1

Sadly, this seems to be the best way to do this:

public static Date convertLocalTimestamp(long millis)
{
    TimeZone tz = TimeZone.getDefault();
    Calendar c = Calendar.getInstance(tz);
    long localMillis = millis;
    int offset, time;

    c.set(1970, Calendar.JANUARY, 1, 0, 0, 0);

    // Add milliseconds
    while (localMillis > Integer.MAX_VALUE)
    {
        c.add(Calendar.MILLISECOND, Integer.MAX_VALUE);
        localMillis -= Integer.MAX_VALUE;
    }
    c.add(Calendar.MILLISECOND, (int)localMillis);

    // Stupidly, the Calendar will give us the wrong result if we use getTime() directly.
    // Instead, we calculate the offset and do the math ourselves.
    time = c.get(Calendar.MILLISECOND);
    time += c.get(Calendar.SECOND) * 1000;
    time += c.get(Calendar.MINUTE) * 60 * 1000;
    time += c.get(Calendar.HOUR_OF_DAY) * 60 * 60 * 1000;
    offset = tz.getOffset(c.get(Calendar.ERA), c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH), c.get(Calendar.DAY_OF_WEEK), time);

    return new Date(millis - offset);
}

(I know that this is several months past post date, but it's a problem that is very useful to solve when working with text messages on Android. dave's answer is wrong.)

Answer 2

Use a Calendar to get what the offset was at the local Epoch, then add that to the local-epoch timestamp.

public static long getLocalToUtcDelta() {
    Calendar local = Calendar.getInstance();
    local.clear();
    local.set(1970, Calendar.JANUARY, 1, 0, 0, 0);
    return local.getTimeInMillis();
}

public static long converLocalTimeToUtcTime(long timeSinceLocalEpoch) {
    return timeSinceLocalEpoch + getLocalToUtcDelta();
}