Get Greenwich Mean Time

Question 1

double timezoneval = 0.0;
        Calendar cal = Calendar.getInstance();
        cal.set(mn.Year(), mn.Month(), mn.Day(), mn.Hour(), mn.minute(),
                (int) mn.second());
        TimeZone tz = TimeZone.getDefault();
        boolean t = tz.inDaylightTime(cal.getTime());
        if (t)
            timezoneval = ((TimeZone.getDefault().getRawOffset())
                    / (60 * 60 * 1000D) + (TimeZone.getDefault()
                    .getDSTSavings() / (60 * 60 * 1000D)));
        else
            timezoneval = ((TimeZone.getDefault().getRawOffset()) / (60 * 60 * 1000D));

You can try this code. This will give you the value of time zone. If you want different timezone value; just change TimeZone tz = TimeZone.getDefault(); to

TimeZone tz = TimeZone.getTimeZone("Asia/Kolkata");
        TimeZone.setDefault(tz);

It will work for you.

Question 2

Check the Timezone class. The raw offset is what you want, in millisecond. You can easily convert it to value in hours.

long rawOffset = TimeZone.getDefault().getRawOffset();

See http://developer.android.com/reference/java/util/TimeZone.html#getRawOffset()

Question 3

tl;dr

how can a get this value using device time zone

ZonedDateTime.
now(
    ZoneId.systemDefault()  // Or: ZoneId.of("Asia/Kolkata") 
)
.getOffset()
.toString()

See this code run live at IdeOne.com.

+05:30

java.time

Solution using java.time, the modern API:

import java.time.ZoneId;
import java.time.ZonedDateTime;

public class Main {
    public static void main(String[] args) {
        ZonedDateTime zdt = ZonedDateTime.now(ZoneId.of("Asia/Kolkata"));
        int seconds = zdt.getOffset().getTotalSeconds();
        System.out.println(seconds);

        // If required, convert it into hours
        double hours = seconds / 3600.0;
        System.out.println(hours);
    }
}

Output:

19800
5.5

Learn more about java.time, the modern date-time API^* from Trail: Date Time.

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}

Question 4

Try:

TimeZone.getDefault();

getDefault() returns the timezone details of the current device. You can then call getOffset() to get the difference between UTC and this time zone.

Question 5

try this code

 public class Sample {


public static void main(String[] args) {
    // TODO Auto-generated method stub
    try {
        String dateStr = "2010-06-14 02:21:49-0400";
        SimpleDateFormat sdf =  new SimpleDateFormat("yyyy-MM-dd HH:mm:ssZ");
        TimeZone tz = TimeZone.getDefault();
        sdf.setTimeZone(tz);
        Date date = sdf.parse(dateStr);

        sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss a");
        String newDateStr = sdf.format(date);

        System.out.println(newDateStr);
    } catch (ParseException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
}

output should like this

2010-06-14 08:21:49 AM

Question 6

This is actually what i need.

double rawOffset = TimeZone.getDefault().getRawOffset();

rawOffset = rawOffset / (60 * 60 * 1000);

System.out.println("rawOffset : " + rawOffset);