Java Data Type - 如何以UTC或GMT获取当前日期和时间

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

public class Main {
    public static void main (String[] args){
        System.out.println(my_time_in("GMT-5:00", "MM/dd/yyyy HH:mm:ss") );
        System.out.println(my_time_in("GMT+5:30", "HH:mm a z 'on' MM/dd/yyyy"));

        System.out.println(my_time_in("America/Los_Angeles", "HH:mm a z 'on' MM/dd/yyyy") );
        System.out.println(my_time_in("America/Buenos_Aires", "HH:mm a z 'on' MM/dd/yyyy") );
    }

    public static String my_time_in(String target_time_zone, String format){
        TimeZone tz = TimeZone.getTimeZone(target_time_zone);
        Date date = Calendar.getInstance().getTime();
        SimpleDateFormat date_format_gmt = new SimpleDateFormat(format);
        date_format_gmt.setTimeZone(tz);
        return date_format_gmt.format(date);
    }
}