Android Time, TimeZone and GPS Locations

I am currently working on an app that shows the planets of the solar system depending on the user's location (longitude and latitude, usually acquired from GPS) and time. These planets (as far as I can tell, its not my original code) depend on a UTC timestamp and as a result if the user is in GMT (aka UTC) the planets appear fine.

But as users go further around the world, China and the US in particular, the planets have been appearing in the wrong places (the Sun is the most obvious one - ignore the fact its a Star). I seem to be getting the time passed to the planet position calculation incorrect and I'm not sure why.

I've had various versions out but none seem to work so far and its almost impossible to tell if something works until I send it out and get an e-mail back telling me I'm wrong. We thought it might be a GSM/CDMA conflict but it doesn't seem this way.

Below is the original code to create a Calendar with GMT time:

public static Calendar convertToGmt(Calendar cal)
{
    Date date = cal.get开发者_StackOverflow中文版Time();
    TimeZone tz = cal.getTimeZone();

    //log.debug("input calendar has date [" + date + "]");

    //Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT 
    long msFromEpochGmt = date.getTime();

    //gives you the current offset in ms from GMT at the current date
    int offsetFromUTC = tz.getOffset(msFromEpochGmt);
    //log.debug("offset is " + offsetFromUTC);

    //create a new calendar in GMT timezone, set to this date and add the offset
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.setTime(date);
    gmtCal.add(Calendar.MILLISECOND, offsetFromUTC);

    //log.debug("Created GMT cal with date [" + gmtCal.getTime() + "]");

    return gmtCal;
}

This was later changed to:

public static Calendar convertToGmt(Calendar cal)
{
    Calendar gmtCal = new GregorianCalendar(TimeZone.getTimeZone("GMT"));
    long time = cal.getTimeInMillis();
    long offset = cal.getTimeZone().getRawOffset();
    gmtCal.setTimeInMillis(time - offset);
    return gmtCal;
}

And the latest version:

public static Calendar convertToGmt(Calendar cal)
{       
    TimeZone timezone = TimeZone.getDefault();
    TimeZone utcTimeZone = TimeZone.getTimeZone("UTC");

    int currentGMTOffset = timezone.getOffset(cal.getTimeInMillis());
    int gmtOffset = utcTimeZone.getOffset(cal.getTimeInMillis());

    cal.setTimeInMillis(cal.getTimeInMillis() + (gmtOffset - currentGMTOffset));
    return cal;
}

In the first two versions a Calendar instance is being passed back whereas the third version (in order to optimize it) merely updates a static instance of it. Tinkering this morning I'm thinking maybe using System.currentTimeInMillis, i.e.:

private static Calendar utc = new GregorianCalendar(TimeZone.getTimeZone("UTC"));
private static Calendar cal = Calendar.getInstance();
public static void convertToGmt()
{           
    cal.setTimeInMillis(System.currentTimeMillis());
    utc.setTimeInMillis(cal.getTimeInMillis());
}

but I'm not sure this makes any difference.

I really am getting quite lost with this - can someone explain to me either where I'm going wrong or how I should approach the problem? I'm hoping several other pairs of eyes might help! :)

---

Why does the timezone matter if the app accepts longitude and latitude? My approach would be to only use the Date, Time and longitude/latitude to figure out the position of planets. java.util.Date stores the time in GMT internally, so converting to GMT manually seems unnecessary.

---

Try this code for use Google Time Zone API to get the current time:

  String get_xml_server_reponse(String server_url){

    URL xml_server = null;

    String xmltext = "";

    InputStream input;


    try {
        xml_server = new URL(server_url);


        try {
            input = xml_server.openConnection().getInputStream();


            final BufferedReader reader = new BufferedReader(new InputStreamReader(input));
            final StringBuilder sBuf = new StringBuilder();

            String line = null;
            try {
                while ((line = reader.readLine()) != null) 
                {
                    sBuf.append(line);
                }
               } 
            catch (IOException e) 
              {
                    Log.e(e.getMessage(), "XML parser, stream2string 1");
              } 
            finally {
                try {
                    input.close();
                    }
                catch (IOException e) 
                {
                    Log.e(e.getMessage(), "XML parser, stream2string 2");
                }
            }

            xmltext =  sBuf.toString();

        } catch (IOException e1) {

                e1.printStackTrace();
            }


        } catch (MalformedURLException e1) {

          e1.printStackTrace();
        }

     return  xmltext;

  }     


 private String get_UTC_Datetime_from_timestamp(long timeStamp){

    try{

        Calendar cal = Calendar.getInstance();
        TimeZone tz = cal.getTimeZone();

        int tzt = tz.getOffset(System.currentTimeMillis());

        timeStamp -= tzt;

        // DateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss",Locale.getDefault());
        DateFormat sdf = new SimpleDateFormat();
        Date netDate = (new Date(timeStamp));
        return sdf.format(netDate);
    }
    catch(Exception ex){
        return "";
     }
    } 

 class NTP_UTC_Time
 {
     private static final String TAG = "SntpClient";

     private static final int RECEIVE_TIME_OFFSET = 32;
     private static final int TRANSMIT_TIME_OFFSET = 40;
     private static final int NTP_PACKET_SIZE = 48;

     private static final int NTP_PORT = 123;
     private static final int NTP_MODE_CLIENT = 3;
     private static final int NTP_VERSION = 3;

     // Number of seconds between Jan 1, 1900 and Jan 1, 1970
     // 70 years plus 17 leap days
     private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;

     private long mNtpTime;

     public boolean requestTime(String host, int timeout) {
         try {
             DatagramSocket socket = new DatagramSocket();
             socket.setSoTimeout(timeout);
             InetAddress address = InetAddress.getByName(host);
             byte[] buffer = new byte[NTP_PACKET_SIZE];
             DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);

             buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);

             writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET);

             socket.send(request);

             // read the response
             DatagramPacket response = new DatagramPacket(buffer, buffer.length);
             socket.receive(response);          
             socket.close();

             mNtpTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);            
         } catch (Exception e) {
           //  if (Config.LOGD) Log.d(TAG, "request time failed: " + e);
             return false;
         }

         return true;
     }


     public long getNtpTime() {
         return mNtpTime;
     }


     /**
      * Reads an unsigned 32 bit big endian number from the given offset in the buffer.
      */
     private long read32(byte[] buffer, int offset) {
         byte b0 = buffer[offset];
         byte b1 = buffer[offset+1];
         byte b2 = buffer[offset+2];
         byte b3 = buffer[offset+3];

         // convert signed bytes to unsigned values
         int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0);
         int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1);
         int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2);
         int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3);

         return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3;
     }

     /**
      * Reads the NTP time stamp at the given offset in the buffer and returns 
      * it as a system time (milliseconds since January 1, 1970).
      */    
     private long readTimeStamp(byte[] buffer, int offset) {
         long seconds = read32(buffer, offset);
         long fraction = read32(buffer, offset + 4);
         return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);        
     }

     /**
      * Writes 0 as NTP starttime stamp in the buffer. --> Then NTP returns Time OFFSET since 1900
      */    
     private void writeTimeStamp(byte[] buffer, int offset) {        
         int ofs =  offset++;

         for (int i=ofs;i<(ofs+8);i++)
           buffer[i] = (byte)(0);             
     }

 }

 String get_time_zone_time(GeoPoint gp){

        String erg = "";
        String raw_offset = "";
        String dst_offset = "";

        double Longitude = gp.getLongitudeE6()/1E6;
        double Latitude = gp.getLatitudeE6()/1E6;

        // String request = "http://ws.geonames.org/timezone?lat="+Latitude+"&lng="+ Longitude+ "&style=full";


        long tsLong = 0; // System.currentTimeMillis()/1000;

        NTP_UTC_Time client = new NTP_UTC_Time();

        if (client.requestTime("pool.ntp.org", 2000)) {              
          tsLong = client.getNtpTime();
        }

        if (tsLong != 0)
        {

        tsLong = tsLong  / 1000;

        // https://maps.googleapis.com/maps/api/timezone/xml?location=39.6034810,-119.6822510&timestamp=1331161200&sensor=true

        String request = "https://maps.googleapis.com/maps/api/timezone/xml?location="+Latitude+","+ Longitude+ "&timestamp="+tsLong +"&sensor=true";

        String xmltext = get_xml_server_reponse(request);

        if(xmltext.compareTo("")!= 0)
        {

         int startpos = xmltext.indexOf("<TimeZoneResponse");
         xmltext = xmltext.substring(startpos);



        XmlPullParser parser;
        try {
            parser = XmlPullParserFactory.newInstance().newPullParser();


             parser.setInput(new StringReader (xmltext));

             int eventType = parser.getEventType();  

             String tagName = "";


             while(eventType != XmlPullParser.END_DOCUMENT) {
                 switch(eventType) {

                     case XmlPullParser.START_TAG:

                           tagName = parser.getName();

                         break;


                     case XmlPullParser.TEXT :


                        if  (tagName.equalsIgnoreCase("raw_offset"))
                          if(raw_offset.compareTo("")== 0)                               
                            raw_offset = parser.getText();  

                        if  (tagName.equalsIgnoreCase("dst_offset"))
                          if(dst_offset.compareTo("")== 0)
                            dst_offset = parser.getText();  


                        break;   

                 }

                 try {
                        eventType = parser.next();
                    } catch (IOException e) {

                        e.printStackTrace();
                    }

                }

                } catch (XmlPullParserException e) {

                    e.printStackTrace();
                    erg += e.toString();
                }

        }      

        int ro = 0;
        if(raw_offset.compareTo("")!= 0)
        { 
            float rof = str_to_float(raw_offset);
            ro = (int)rof;
        }

        int dof = 0;
        if(dst_offset.compareTo("")!= 0)
        { 
            float doff = str_to_float(dst_offset);
            dof = (int)doff;
        }

        tsLong = (tsLong + ro + dof) * 1000;



        erg = get_UTC_Datetime_from_timestamp(tsLong);
        }


  return erg;

}

And use it with:

GeoPoint gp = new GeoPoint(39.6034810,-119.6822510);
String Current_TimeZone_Time = get_time_zone_time(gp);