對於手機，如果有SIM卡支援的話，在設定時間時可以通過選擇自動從網路獲取時間來取得當地的時間和時區：

但如果手機沒有SIM卡，此時如果有Wifi連結，手機依然可以通過網路自動獲取時間（時區此時需要手動設定）。 檢視Android原始碼，在android.net 中有 SntpClient類，可以通過訪問NTP伺服器來取得當前的GMT時間。pool.ntp.org為最常用的一個NTF伺服器。修改SntpClient程式碼，你也可以在自己的應用（包括非Android應用）中通過NTP伺服器來取得當前GMT時間，程式碼如下：

import java.net.DatagramPacket;import java.net.DatagramSocket;import
 
 java.net.InetAddress;import java.util.Date; public class GetTime {     public static void main(String[] args) {        SntpClient client = new SntpClient();        if (client.requestTime("pool.ntp.org", 30000)) {            long now = client.getNtpTime() + System.nanoTime() / 1000                    - client.getNtpTimeReference();            Date current = new
 
 Date(now);            System.out.println(current.toString());        }     }} class SntpClient {     private static final int ORIGINATE_TIME_OFFSET = 24;    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;     // system time computed from NTP server response    private long mNtpTime;     // value of SystemClock.elapsedRealtime() corresponding to mNtpTime    private long mNtpTimeReference;     // round trip time in milliseconds    private long mRoundTripTime;     /**     * Sends an SNTP request to the given host and processes the response.     *     * @param host     *            host name of the server.     * @param timeout     *            network timeout in milliseconds.     * @return true if the transaction was successful.     */    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);             // set mode = 3 (client) and version = 3            // mode is in low 3 bits of first byte            // version is in bits 3-5 of first byte            buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);             // get current time and write it to the request packet            long requestTime = System.currentTimeMillis();            long requestTicks = System.nanoTime() / 1000;            writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);             socket.send(request);             // read the response            DatagramPacket response = new DatagramPacket(buffer, buffer.length);            socket.receive(response);            long responseTicks = System.nanoTime() / 1000;            long responseTime = requestTime + (responseTicks - requestTicks);            socket.close();             // extract the results            long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);            long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);            long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);            long roundTripTime = responseTicks - requestTicks                    - (transmitTime - receiveTime);            // receiveTime = originateTime + transit + skew            // responseTime = transmitTime + transit - skew            // clockOffset = ((receiveTime - originateTime) + (transmitTime -            // responseTime))/2            // = ((originateTime + transit + skew - originateTime) +            // (transmitTime - (transmitTime + transit - skew)))/2            // = ((transit + skew) + (transmitTime - transmitTime - transit +            // skew))/2            // = (transit + skew - transit + skew)/2            // = (2 * skew)/2 = skew            long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime)) / 2;            // if (Config.LOGD) Log.d(TAG, "round trip: " + roundTripTime +            // " ms");            // if (Config.LOGD) Log.d(TAG, "clock offset: " + clockOffset +            // " ms");             // save our results - use the times on this side of the network            // latency            // (response rather than request time)            mNtpTime = responseTime + clockOffset;            mNtpTimeReference = responseTicks;            mRoundTripTime = roundTripTime;        } catch (Exception e) {             return false;        }         return true;    }     /**     * Returns the time computed from the NTP transaction.     *     * @return time value computed from NTP server response.     */    public long getNtpTime() {        return mNtpTime;    }     /**     * Returns the reference clock value (value of     * SystemClock.elapsedRealtime()) corresponding to the NTP time.     *     * @return reference clock corresponding to the NTP time.     */    public long getNtpTimeReference() {        return mNtpTimeReference;    }     /**     * Returns the round trip time of the NTP transaction     *     * @return round trip time in milliseconds.     */    public long getRoundTripTime() {        return mRoundTripTime;    }     /**     * 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 << <img class="wp-smiley" alt="8)" src="http://www.imobilebbs.com/wordpress/wp-includes/images/smilies/icon_cool.gif">                + (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 system time (milliseconds since January 1, 1970) as an NTP time     * stamp at the given offset in the buffer.     */    private void writeTimeStamp(byte[] buffer, int offset, long time) {        long seconds = time / 1000L;        long milliseconds = time - seconds * 1000L;        seconds += OFFSET_1900_TO_1970;         // write seconds in big endian format        buffer[offset++] = (byte) (seconds >> 24);        buffer[offset++] = (byte) (seconds >> 16);        buffer[offset++] = (byte) (seconds >> 8);        buffer[offset++] = (byte) (seconds >> 0);         long fraction = milliseconds * 0x100000000L / 1000L;        // write fraction in big endian format        buffer[offset++] = (byte) (fraction >> 24);        buffer[offset++] = (byte) (fraction >> 16);        buffer[offset++] = (byte) (fraction >> 8);        // low order bits should be random data        buffer[offset++] = (byte) (Math.random() * 255.0);    }}

執行這個Java應用，可以得到當前GMT時間，如：Sat Jun 16 10:52:19 BST 2012