View source for Arduino: RTC DS1302 NTP Sync

==Code==

 // time ntp RTC DS1302 set
 
 #include <Time.h>
 #include <SPI.h>
 #include <Ethernet.h>
 #include <EthernetUdp.h>
 #include <DS1302.h>
 
 // Init the DS1302
 // DS1302 rtc([CE/RST], [I/O], [CLOCK]);
 DS1302 rtc(8, 7, 6);
 
 // Ethernet library configuration
 byte mac[] = { 0x24, 0x7B, 0xA2, 0x4A, 0x52, 0x10 };
 IPAddress timeServer(128, 138, 140, 44 ); // time.nist.gov
 
 unsigned int localPort = 8888;
 const int NTP_PACKET_SIZE= 48;     
 byte packetBuffer[NTP_PACKET_SIZE]; 
 EthernetUDP Udp;                   
 
 void setup() {
   // Set the clock to run-mode, and disable the write protection
   rtc.halt(false);
   rtc.writeProtect(false);
 
   // Setup Communication
   Serial.begin(9600);
   Ethernet.begin(mac);
   Udp.begin(localPort);
 
   Serial.print("get time");
   setSyncInterval(10); // Set seconds between re-sync
   getNtpTime;
 
   // The following lines can be commented out to use the values already stored in the DS1302
   rtc.setDOW(WEDNESDAY);        // Set Day-of-Week to MONDAY
   // rtc.setTime(13, 0, 0);     // Set the time to 12:00:00 (24hr format)
   rtc.setDate(30, 5, 2018);   // Set the date to Feb 2, 2016
 }
 
 void loop() {
   setSyncProvider(getNtpTime);
 
   rtc.setTime(hour()+7, minute(), second());
 
   // Send Day-of-Week
   Serial.print(rtc.getDOWStr());
   Serial.print(" ");
  
   // Send date
   Serial.print(rtc.getDateStr());
   Serial.print(" -- ");
  
   // Send time
   Serial.println(rtc.getTimeStr());
   
   getNtpTime;
   // delay(1000);
 }  
 
 /*-------- NTP code ----------*/
 
 unsigned long getNtpTime()
 {
   sendNTPpacket(timeServer); // send an NTP packet to a time server
     delay(500);
   
     if ( Udp.parsePacket() ) {
      Udp.read(packetBuffer,NTP_PACKET_SIZE);
      unsigned long hi = word(packetBuffer[40], packetBuffer[41]);
      unsigned long low = word(packetBuffer[42], packetBuffer[43]);
      unsigned long secsSince1900 = hi << 16 | low; 
      const unsigned long seventyYears = 2208988800UL;     
      unsigned long epoch = secsSince1900 - seventyYears;
      return epoch;
   }
   return 0; // return 0 if unable to get the time
 }
 
 unsigned long sendNTPpacket(IPAddress& address)
 {
   memset(packetBuffer, 0, NTP_PACKET_SIZE);  // set all bytes in the buffer to 0
 
   // Initialize values needed to form NTP request
   packetBuffer[0] = B11100011;   // LI, Version, Mode
   packetBuffer[1] = 0;     // Stratum
   packetBuffer[2] = 6;     // Max Interval between messages in seconds
   packetBuffer[3] = 0xEC;  // Clock Precision
   // bytes 4 - 11 are for Root Delay and Dispersion and were set to 0 by memset
   packetBuffer[12]  = 49;  // four-byte reference ID identifying
   packetBuffer[13]  = 0x4E;
   packetBuffer[14]  = 49;
   packetBuffer[15]  = 52;
 
   // send the packet requesting a timestamp:
   Udp.beginPacket(address, 123); //NTP requests are to port 123
   Udp.write(packetBuffer,NTP_PACKET_SIZE);
   Udp.endPacket();
 }