Arduino NTP for IST using latest Time.h

/*

  • Time_NTP.pde
  • Example showing time sync to NTP time source
  • This sketch uses the Ethernet library
    */

#include <Time.h>
#include <Ethernet.h>
#include <EthernetUdp.h>
#include <SPI.h>

byte mac = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
// NTP Servers:
IPAddress timeServer(120,88,46,10); // 0.in.pool.ntp.org

const int timeZoneOffset = 19800;

EthernetUDP Udp;
unsigned int localPort = 8888; // local port to listen for UDP packets
int hr;
void setup()
{
Serial.begin(9600);

//while (!Serial) ; // Needed for Leonardo only
delay(250);
Serial.println(“TimeNTP Example”);
if (Ethernet.begin(mac) == 0) {
// no point in carrying on, so do nothing forevermore:
while (1) {
Serial.println(“Failed to configure Ethernet using DHCP”);
delay(10000);
}
}
Serial.print("IP number assigned by DHCP is ");
Serial.println(Ethernet.localIP());
Udp.begin(localPort);
Serial.println(“waiting for sync”);
setSyncProvider(getNtpTime);
}

time_t prevDisplay = 0; // when the digital clock was displayed

void loop()
{

if (timeStatus() != timeNotSet) {
if (now() != prevDisplay) { //update the display only if time has changed
prevDisplay = now();
digitalClockDisplay();
}
}
}

void digitalClockDisplay(){
// digital clock display of the time
//Serial.print(hour());

printHrs(hour());
printMins(minute());
printSecs(second());
Serial.print(" “);
Serial.print(day());
Serial.print(” “);
Serial.print(month());
Serial.print(” ");
Serial.print(year());
Serial.println();
}

void printHrs(int digits){
// utility for digital clock display: prints preceding colon and leading 0
if(digits < 10)
Serial.print(‘0’);
Serial.print(digits);
}

void printMins(int digits){
// utility for digital clock display: prints preceding colon and leading 0
Serial.print(":");
if(digits < 10)
Serial.print(‘0’);
Serial.print(digits);
}

void printSecs(int digits){
// utility for digital clock display: prints preceding colon and leading 0
Serial.print(":");
if(digits < 10)
Serial.print(‘0’);
Serial.print(digits);
}

/-------- NTP code ----------/

const int NTP_PACKET_SIZE = 48; // NTP time is in the first 48 bytes of message
byte packetBuffer[NTP_PACKET_SIZE]; //buffer to hold incoming & outgoing packets

time_t getNtpTime()
{
while (Udp.parsePacket() > 0) ; // discard any previously received packets
Serial.println(“Transmit NTP Request”);
sendNTPpacket(timeServer);
uint32_t beginWait = millis();
while (millis() - beginWait < 1500) {
int size = Udp.parsePacket();
if (size >= NTP_PACKET_SIZE) {
Serial.println(“Receive NTP Response”);
Udp.read(packetBuffer, NTP_PACKET_SIZE); // read packet into the buffer
unsigned long secsSince1900;
// convert four bytes starting at location 40 to a long integer
secsSince1900 = (unsigned long)packetBuffer[40] << 24;
secsSince1900 |= (unsigned long)packetBuffer[41] << 16;
secsSince1900 |= (unsigned long)packetBuffer[42] << 8;
secsSince1900 |= (unsigned long)packetBuffer[43];
return secsSince1900 - 2208988800UL + timeZoneOffset;
}
}
Serial.println(“No NTP Response :-(”);
return 0; // return 0 if unable to get the time
}

// send an NTP request to the time server at the given address
void sendNTPpacket(IPAddress &address)
{
// set all bytes in the buffer to 0
memset(packetBuffer, 0, NTP_PACKET_SIZE);
// Initialize values needed to form NTP request
// (see URL above for details on the packets)
packetBuffer[0] = 0b11100011; // LI, Version, Mode
packetBuffer[1] = 0; // Stratum, or type of clock
packetBuffer[2] = 6; // Polling Interval
packetBuffer[3] = 0xEC; // Peer Clock Precision
// 8 bytes of zero for Root Delay & Root Dispersion
packetBuffer[12] = 49;
packetBuffer[13] = 0x4E;
packetBuffer[14] = 49;
packetBuffer[15] = 52;
// all NTP fields have been given values, now
// you can send a packet requesting a timestamp:
Udp.beginPacket(address, 123); //NTP requests are to port 123
Udp.write(packetBuffer, NTP_PACKET_SIZE);
Udp.endPacket();
}

//How to calculate timeZoneOffset? GMT Timezone * 3600

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Also. Do you know what GMT stands for ?
It should be self explanatory.
Where is Greenwich ?