Java 本地時間與UTC時間轉換

UTC時間：通用協調時(Universal Time Coordinated),與格林尼治平均時(GMT,Greenwich Mean Time)一樣，都是與英國倫敦的本地時相同。

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class GetUTCTime {

    // 取得本地時間：
    private Calendar cal = Calendar.getInstance();
    // 取得時間偏移量：
    private int zoneOffset = cal.get(java.util.Calendar.ZONE_OFFSET);
    // 取得夏令時差：
    private int dstOffset = cal.get(java.util.Calendar.DST_OFFSET);

    public static void main(String[] args) {
        GetUTCTime gc = new GetUTCTime();
        long mill = gc.getUTCTimeStr();
        gc.setUTCTime(mill);

    }

    public long getUTCTimeStr() {

        System.out.println("local millis = " + cal.getTimeInMillis()); // 等效System.currentTimeMillis() , 統一值，不分時區

        // 從本地時間里扣除這些差量，即可以取得UTC時間：
        cal.add(java.util.Calendar.MILLISECOND, -(zoneOffset + dstOffset));

        long mills = cal.getTimeInMillis();
        System.out.println("UTC = " + mills);

        return mills;
    }

    public void setUTCTime(long millis) {

        cal.setTimeInMillis(millis);

        SimpleDateFormat foo = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String time = foo.format(cal.getTime());
        System.out.println("GMT time= " + time);

        // 從本地時間里扣除這些差量，即可以取得UTC時間：
        cal.add(java.util.Calendar.MILLISECOND, (zoneOffset + dstOffset));
        time = foo.format(cal.getTime());
        System.out.println("Local time = " + time);

    }

    public void getGMTTime() {
        //mothed 2
        TimeZone gmtTime = TimeZone.getTimeZone("GMT");
        Date date = new Date();
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        format.setTimeZone(gmtTime);
        System.out.println("GMT Time: " + format.format(date));
        
        //method 2
        Calendar calendar1 = Calendar.getInstance();
        calendar1.setTimeZone(gmtTime);
        //System.out.println(calendar1.getTime());    //時區無效
        //System.out.println(calendar1.getTimeInMillis()); //時區無效
        System.out.println("GMT hour = " + calendar1.get(Calendar.HOUR));
    }
}