java在java中转换毫秒日期对应的时区日期

Question

1)I need to find out the user timezone and time. For that I use

Calendar currentdate1 = Calendar.getInstance();
TimeZone tz = Calendar.getInstance().getTimeZone(); 
System.out.println("time zone"+tz);
System.out.println(tz.getDisplayName()); // (Now India Standard Time)  
System.out.println(tz.getID()); // (Now . Asia/Kolkata)

By using this I need to find out the current time of that user1 at corresponding timezone.

2)One user2 upload video on there date. I convert that into millisecond and store it database. User1 want to see the uploading time of user2 as user1 date and time corresponding to his timezone.How can I do this.

Answer 1

Since every time is stored in milliseconds there is no need to do a conversion. What you are after is presenting the time in milliseconds in the proper format. Consider the following scenario:

User2 uploads a video on a date that corresponds to the value t=12345678911ms (this is an absolute value that corresponds to an offset from January 1, 1970, 00:00 and does not need to change)

User1 wants to see which date and time correspond to the value t based on his timezone. So after setting up the proper calendar/timezone

Calendar user1C = Calendar.getInstance();//Executed on the system of user1 this command will get the correct locale and timezone:

You just have to set the time on the calendar of user1 based on the value t of user2:

user1C.setTimeInMillis(t);//Sets the the time of the current calendar based on the value of user2

If now you print the calendar time you will see the date and the time that corresponds to user2

System.out.println(user1C.getTime());

Answer 2

It is important to understand, that a java.util.Date object is an absolute point in time. There is no way to get a timezone from that object. The long value behind java.util.Date can be interpreted to get a date (date string) the way people normally use it. This depends on the timezone the formatter object uses.

You can also get a java.util.Date object from an date string. The result depends on the timezone the calendar object uses for transformation/parsing.

This example might help:

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


public class Test {

    public static void main(final String[] args) {
        Date now = new Date();
        printDateForBerlinAndKolkata(now, "Current Time:");

        Calendar cal1 = new GregorianCalendar(TimeZone.getTimeZone("Europe/Berlin"));
        Calendar cal2 = new GregorianCalendar(TimeZone.getTimeZone("Asia/Kolkata"));

        cal1.set(2014, 11, 5, 15, 30);
        printDateForBerlinAndKolkata(cal1.getTime(), "Gregorian Calendar 2014-12-05 15:30 Timezone for Berlin:");

        cal2.set(2014, 11, 5, 15, 30, 0);
        printDateForBerlinAndKolkata(cal2.getTime(), "Gregorian Calendar 2014-12-05 15:30 Timezone for Kolkata:");
    }

    private static void printDateForBerlinAndKolkata(Date now, String headline) {
        System.out.println(headline);
        System.out.println("---------------------------------------------");
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd HH:mm:ssZ");
        sdf.setTimeZone(TimeZone.getTimeZone("Europe/Berlin"));
        System.out.println("Europe/Berlin: " + sdf.format(now));
        sdf.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));
        System.out.println("Asia/Kolkata:  " + sdf.format(now));
        System.out.println("long value:    " + now.getTime());
        System.out.println("=============================================\n");
    }

}

Output:

Current Time:
---------------------------------------------
Europe/Berlin: 2014 Dez 05 16:35:21+0100
Asia/Kolkata:  2014 Dez 05 21:05:21+0530
long value:    1417793721481
=============================================

Gregorian Calendar 2014-12-05 15:30 Timezone for Berlin:
---------------------------------------------
Europe/Berlin: 2014 Dez 05 15:30:21+0100
Asia/Kolkata:  2014 Dez 05 20:00:21+0530
long value:    1417789821506
=============================================

Gregorian Calendar 2014-12-05 15:30 Timezone for Kolkata:
---------------------------------------------
Europe/Berlin: 2014 Dez 05 11:00:00+0100
Asia/Kolkata:  2014 Dez 05 15:30:00+0530
long value:    1417773600506
=============================================

Perhaps the following link is also of interest: Wikipedia Article on Unix Time