在Java中将本地时间戳转换为UTC时间戳

Question

我有一个毫秒 - 自 - 本地 - 纪元时间戳,我想将其转换为自UTC时间戳的毫秒时间戳.从快速浏览文档看起来像这样的东西会起作用:

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

有一个更好的方法吗？

Answer 1

可悲的是,这似乎是最好的方法:

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);
}

(我知道这是发布日期后的几个月,但是在Android上使用短信时这是一个非常有用的问题.戴夫的回答是错误的.)

Answer 2

使用a Calendar来获取本地Epoch的偏移量,然后将其添加到local-epoch时间戳.

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();
}