How to get time zone from date

Question

I am trying to get time zone from an existing date to use it for some other date conversion. Can someone reply with updating the todos in the below code. Appreciate any help.

Or just to make it simple is there some java api to which i give +0530 and it returns IST :)

Here is my code :

import java.text.SimpleDateFormat
import java.util.*;
import java.text.DateFormat;

SimpleDateFormat isoFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ");
Date date = isoFormat.parse("2016-04-21T00:00:00+0530");

//todo print time zone 
//todo here should print IST since date is having +0530

Answer 1

This is not possible. A Date does not have time zone information attached. It is just a point in time, internally represented as milliseconds since 1.1.1970 midnight UTC (excluding leap seconds).

Answer 2

A java.util.Date does not have a time zone. It is a pure time in UTC. The parser converted the string to the internal value.

A java.time.ZonedDateTime (Java 8+) does have a time zone.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ssZ");
ZonedDateTime dt = ZonedDateTime.parse("2016-04-21T00:00:00+0530", formatter);
ZoneId zone = dt.getZone();

If running Java 6 or 7, use the backport of the Java SE 8 date-time classes .
For Java 5+ use the Joda-Time library .

Answer 3

Just to answer myself so that it might help some one else :

I was having date as string as input lets say :

String startDate = "2016-04-21T00:00:00+0530"
//i can calculate the timezone offset using
String offSet = startDate.substring(startDate.length() - 5) //gives +0530

Method used to calculate timezone. Here we give offset calculated above and the below method returns the TimeZone object:

public static TimeZone fetchTimeZone(String offset) {
        if (offset.length() != 5) {
            return null
        }

        TimeZone tz

        Integer offsetHours = Integer.parseInt(offset.substring(0, 3))
        Integer offsetMinutes = Integer.parseInt(offset.substring(3))

        String[] ids = TimeZone.getAvailableIDs()

        for (int i = 0; i < ids.length; i++) {
            tz = TimeZone.getTimeZone(ids[i])

            long hours = TimeUnit.MILLISECONDS.toHours(tz.getRawOffset())
            long minutes = Math.abs(TimeUnit.MILLISECONDS.toMinutes(tz.getRawOffset()) % 60)

            if (hours != offsetHours || minutes != offsetMinutes) {
                tz = null
            } else {
                break
            }
        }
        return tz
    }

Finally i use the Timezone from above method to format any date to that timezone :

SimpleDateFormat timeZonedFormatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
TimeZone timeZone = fetchTimeZone(offSet) //from above method and offset from first code
timeZonedFormatter.setTimeZone(timeZone);
//this timeZonedFormatter can be used to format any date into the respective timeZone