Showing correct time from Milliseconds with desired TimeZone

I’m developing an application which takes data from Google TimeZone API. Simply I have time in milliseconds of desired place on Earth.

For Example : 1504760156000 it’s showing Date time In London which is

Thu Sep 07 2017 09:55:56

As I’m in TimeZone +05:00 from UTC if I manipulate 1504760156000 these milliseconds it will show me whole date time like below:

Thu Sep 07 2017 09:55:56 GMT+0500 (Pakistan Standard Time)

but I want to show:

Thu Sep 07 2017 09:55:56 GMT+0100 (British Summer Time)

The problem is: I have correct date and time for London but enable to show/change TimeZone without changing Time because Time is correct according to London.

UPDATED

After getting some comments. You are not getting me here in my example.

Suppose I am in Pakistan and Time here is 1:55 PM so I asked GOOGLE API via my application to tell me whats the time in London at moment. Google API tell me time in London is 1504760156000 (9:55 AM) in milliseconds if I convert these milliseconds to Date Object it will print out like below:

Date date =new Date(1504760156000)

Thu Sep 07 2017 09:55:56 GMT+0500 (Pakistan Standard Time)

It will manipulate it according to my Local TimeZone but I want results like below

Thu Sep 07 2017 09:55:56 GMT+0100 (British Summer Time)

Updated 2

I prepared timestamp in seconds in UTC as Google Time Zone API needed timestamp UTC in form of seconds

"https://maps.googleapis.com/maps/api/timezone/json?location="+Latitude +","+Longitude+"&timestamp="+currentTimeUTCinSeonds+"&key="API KEY"

Google API respond me with below JSON against London.

{
    "dstOffset" : 3600,   
    "rawOffset" : 0,  
    "status" : "OK",  
    "timeZoneId" : "Europe/London",  
    "timeZoneName" : "British Summer Time"
}

According to Docs:

Calculating the Local Time

The local time of a given location is the sum of the timestamp parameter, and the dstOffset and rawOffset fields from the result.

I Sum up result timestamp+rawoffset+dstoffset*1000='1504760156000' (at moment when I tried it)

Code from Project

Long ultimateTime=((Long.parseLong(timeObject1.getDstOffset())*1000)+(Long.parseLong(timeObject1.getRawOffset())*1000)+timestamp*1000);
                    timeObject1.setTimestamp(ultimateTime);  //its Sime POJO object to save current time of queried Location 
                    Date date=new Date(ultimateTime);
                    date1.setText("Date Time : "+date);

As I said I’m manipulating result in Local Time Zone so it gave me below result at that time:

Thu Sep 07 2017 09:55:56 GMT+0500 (Pakistan Standard Time)

But I knew API gave me correct time. The problem is Local offset from UTC . I just want to change GMT+0500 to GMT+0100

Answer

Timestamps represent an “absolute” value of a time elapsed since epoch. Your currentTimeUTCinSeconds, for example, represent the number of seconds since unix epoch (which is 1970-01-01T00:00Z, or January 1st 1970 at midnight in UTC). Java API’s usually work with the number of milliseconds since epoch.

But the concept is the same – those values are “absolute”: they are the same for everyone in the world, no matter where they are. If 2 people in different parts of the world (in different timezones) get the current timestamp at the same time, they’ll all get the same number.

What changes is that, in different timezones, this same number represents a different local date and time.

For example, the timestamp you’re using, that corresponds to Sep 7th 2017 08:55:56 UTC, which value is 1504774556 (the number of seconds since epoch). This same number corresponds to 09:55 in London, 13:55 in Karachi, 17:55 in Tokyo and so on. Changing this number will change the local times for everyone – there’s no need to manipulate it.

If you want to get a java.util.Date that represents this instant, just do:

int currentTimeUTCinSeconds = 1504774556;
// cast to long to not lose precision
Date date = new Date((long) currentTimeUTCinSeconds * 1000);

This date will keep the value 1504774556000 (the number of milliseconds since epoch). This value corresponds to 09:55 in London, 13:55 in Karachi and 17:55 in Tokyo.

But printing this date will convert it to your JVM default timezone (here is a good explanation about the behaviour of Date::toString() method). When you do "Date Time : "+date, it implicity calls toString() method, and the result is the date converted to your default timezone.

If you want the date in a specific format and in a specific timezone, you’ll need a SimpleDateFormat. Just printing the date (with System.out.println or by logging it) won’t work: you can’t change the format of the date object itself, because a Date has no format.

I also use a java.util.Locale to specify that the month and day of week must be in English. If you don’t specify a locale, it’ll use the system default, and it’s not guaranteed to always be English (and this can be changed, even at runtime, so it’s better to always specify a locale):

// use the same format, use English for month and day of week
SimpleDateFormat sdf = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'Z (zzzz)", Locale.ENGLISH);
// set the timezone I want
sdf.setTimeZone(TimeZone.getTimeZone("Europe/London"));
// format the date
System.out.println(sdf.format(date));

The output will be:

Thu Sep 07 2017 09:55:56 GMT+0100 (British Summer Time)

Note that I don’t need to manipulate the timestamp value. I don’t use the google API, but I think their explanation is too confusing and the code above achieve the same results with less complication.

In your specific case, you can do:

date1.setText("Date Time : "+sdf.format(date));

Java new Date/Time API

The old classes (Date, Calendar and SimpleDateFormat) have lots of problems and design issues, and they’re being replaced by the new APIs.

In Android you can use the ThreeTen Backport, a great backport for Java 8’s new date/time classes. To make it work, you’ll also need the ThreeTenABP (more on how to use it here).

To get a date from a timestamp, I use a org.threeten.bp.Instant with a org.threeten.bp.ZoneId to convert it to a timezone, creating a org.threeten.bp.ZonedDateTime. Then I use a org.threeten.bp.format.DateTimeFormatter to format it:

int currentTimeUTCinSeconds = 1504774556;
// get the date in London from the timestamp
ZonedDateTime z = Instant.ofEpochSecond(currentTimeUTCinSeconds).atZone(ZoneId.of("Europe/London"));
// format it
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("EEE MMM dd yyyy HH:mm:ss 'GMT'XX (zzzz)", Locale.ENGLISH);
System.out.println(fmt.format(z));

The output is the same:

Thu Sep 07 2017 09:55:56 GMT+0100 (British Summer Time)

In your case, just do:

date1.setText("Date Time : "+fmt.format(z));