Using an offset to calculate Timezone is a wrong approach, and you will always encounter problems. Time zones and daylight saving rules may change on several occasions during a year, and It's difficult to keep up with changes.
To get the system's IANA timezone in JavaScript, you should use
console.log(Intl.DateTimeFormat().resolvedOptions().timeZone)
Old compatibility information
ecma-402/1.0 says that timeZone
may be undefined if not provided to constructor. However, future draft (3.0) fixed that issue by changing to system default timezone.
In this version of the ECMAScript Internationalization API, the
timeZone
property will remain undefined if no timeZone
property was
provided in the options object provided to the Intl.DateTimeFormat
constructor. However, applications should not rely on this, as future
versions may return a String value identifying the host environment’s
current time zone instead.
in ecma-402/3.0 which is still in a draft it changed to
In this version of the ECMAScript 2015 Internationalization API, the
timeZone
property will be the name of the default time zone if no
timeZone
property was provided in the options object provided to the
Intl.DateTimeFormat
constructor. The previous version left the
timeZone
property undefined in this case.
Here's what I do:
- First of all I check what providers are enabled. Some may be disabled on the device, some may be disabled in application manifest.
- If any provider is available I start location listeners and timeout timer. It's 20 seconds in my example, may not be enough for GPS so you can enlarge it.
- If I get update from location listener I use the provided value. I stop listeners and timer.
- If I don't get any updates and timer elapses I have to use last known values.
- I grab last known values from available providers and choose the most recent of them.
Here's how I use my class:
LocationResult locationResult = new LocationResult(){
@Override
public void gotLocation(Location location){
//Got the location!
}
};
MyLocation myLocation = new MyLocation();
myLocation.getLocation(this, locationResult);
And here's MyLocation class:
import java.util.Timer;
import java.util.TimerTask;
import android.content.Context;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
public class MyLocation {
Timer timer1;
LocationManager lm;
LocationResult locationResult;
boolean gps_enabled=false;
boolean network_enabled=false;
public boolean getLocation(Context context, LocationResult result)
{
//I use LocationResult callback class to pass location value from MyLocation to user code.
locationResult=result;
if(lm==null)
lm = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
//exceptions will be thrown if provider is not permitted.
try{gps_enabled=lm.isProviderEnabled(LocationManager.GPS_PROVIDER);}catch(Exception ex){}
try{network_enabled=lm.isProviderEnabled(LocationManager.NETWORK_PROVIDER);}catch(Exception ex){}
//don't start listeners if no provider is enabled
if(!gps_enabled && !network_enabled)
return false;
if(gps_enabled)
lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListenerGps);
if(network_enabled)
lm.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, locationListenerNetwork);
timer1=new Timer();
timer1.schedule(new GetLastLocation(), 20000);
return true;
}
LocationListener locationListenerGps = new LocationListener() {
public void onLocationChanged(Location location) {
timer1.cancel();
locationResult.gotLocation(location);
lm.removeUpdates(this);
lm.removeUpdates(locationListenerNetwork);
}
public void onProviderDisabled(String provider) {}
public void onProviderEnabled(String provider) {}
public void onStatusChanged(String provider, int status, Bundle extras) {}
};
LocationListener locationListenerNetwork = new LocationListener() {
public void onLocationChanged(Location location) {
timer1.cancel();
locationResult.gotLocation(location);
lm.removeUpdates(this);
lm.removeUpdates(locationListenerGps);
}
public void onProviderDisabled(String provider) {}
public void onProviderEnabled(String provider) {}
public void onStatusChanged(String provider, int status, Bundle extras) {}
};
class GetLastLocation extends TimerTask {
@Override
public void run() {
lm.removeUpdates(locationListenerGps);
lm.removeUpdates(locationListenerNetwork);
Location net_loc=null, gps_loc=null;
if(gps_enabled)
gps_loc=lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);
if(network_enabled)
net_loc=lm.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
//if there are both values use the latest one
if(gps_loc!=null && net_loc!=null){
if(gps_loc.getTime()>net_loc.getTime())
locationResult.gotLocation(gps_loc);
else
locationResult.gotLocation(net_loc);
return;
}
if(gps_loc!=null){
locationResult.gotLocation(gps_loc);
return;
}
if(net_loc!=null){
locationResult.gotLocation(net_loc);
return;
}
locationResult.gotLocation(null);
}
}
public static abstract class LocationResult{
public abstract void gotLocation(Location location);
}
}
Somebody may also want to modify my logic. For example if you get update from Network provider don't stop listeners but continue waiting. GPS gives more accurate data so it's worth waiting for it. If timer elapses and you've got update from Network but not from GPS then you can use value provided from Network.
One more approach is to use LocationClient http://developer.android.com/training/location/retrieve-current.html. But it requires Google Play Services apk to be installed on user device.
Best Answer
Time Zone Location Web Services
Raw Time Zone Boundary Data
The following projects have previously been sources of time zone boundary data, but are no longer actively maintained.
Time Zone Geolocation Offline Implementations
Implementations that use the Timezone Boundary Builder data
Implementations that use the older tz_world data
Libraries that call one of the web services
Self-hosted web services
Other Ideas
Please update this list if you know of any others
Also, note that the nearest-city approach may not yield the "correct" result, just an approximation.
Conversion To Windows Zones
Most of the methods listed will return an IANA time zone id. If you need to convert to a Windows time zone for use with the
TimeZoneInfo
class in .NET, use the TimeZoneConverter library.Don't use zone.tab
The tz database includes a file called
zone.tab
. This file is primarily used to present a list of time zones for a user to pick from. It includes the latitude and longitude coordinates for the point of reference for each time zone. This allows a map to be created highlighting these points. For example, see the interactive map shown on the moment-timezone home page.While it may be tempting to use this data to resolve the time zone from a latitude and longitude coordinates, consider that these are points - not boundaries. The best one could do would be to determine the closest point, which in many cases will not be the correct point.
Consider the following example:
The two squares represent different time zones, where the black dot in each square is the reference location, such as what can be found in zone.tab. The blue dot represents the location we are attempting to find a time zone for. Clearly, this location is within the orange zone on the left, but if we just look at closest distance to the reference point, it will resolve to the greenish zone on the right.