time.cpp

#include "time.h"

unsigned int localPort = 2390;      // local port to listen for UDP packets
IPAddress timeServer(129, 6, 15, 28); // time.nist.gov NTP server
const int NTP_PACKET_SIZE = 48; // NTP time stamp is in the first 48 bytes of the message
byte packetBuffer[ NTP_PACKET_SIZE]; //buffer to hold incoming and outgoing packets
WiFiUDP Udp; // A UDP instance to let us send and receive packets over UDP

const int GMT = 1; //change this to adapt it to your time zone

void setTimeUsingTimeServer(RTCZero rtc) {
  unsigned long epoch;
  int numberOfTries = 0, maxTries = 6;
  do {
    epoch = readLinuxEpochUsingNTP();
    numberOfTries++;
  }
  while ((epoch == 0) || (numberOfTries > maxTries));

  if (numberOfTries > maxTries) {
    Serial.print("NTP unreachable!!");
    while (1);
  }
  else {
    Serial.print("Epoch received: ");
    Serial.println(epoch);
    rtc.setEpoch(epoch);

    Serial.println();
  }
}

// http://www.arduino.cc/en/Tutorial/WiFiRTC
unsigned long readLinuxEpochUsingNTP()
{
  Udp.begin(localPort);
  sendNTPpacket(timeServer); // send an NTP packet to a time server
  // wait to see if a reply is available
  delay(1000);

  if ( Udp.parsePacket() ) {
    Serial.println("NTP time received");
    // We've received a packet, read the data from it
    Udp.read(packetBuffer, NTP_PACKET_SIZE); // read the packet into the buffer

    //the timestamp starts at byte 40 of the received packet and is four bytes,
    // or two words, long. First, esxtract the two words:

    unsigned long highWord = word(packetBuffer[40], packetBuffer[41]);
    unsigned long lowWord = word(packetBuffer[42], packetBuffer[43]);
    // combine the four bytes (two words) into a long integer
    // this is NTP time (seconds since Jan 1 1900):
    unsigned long secsSince1900 = highWord << 16 | lowWord;

    // now convert NTP time into everyday time:
    // Unix time starts on Jan 1 1970. In seconds, that's 2208988800:
    const unsigned long seventyYears = 2208988800UL;
    // subtract seventy years:

    Udp.stop();
    return (secsSince1900 - seventyYears);
  }

  else {
    Udp.stop();
    return 0;
  }
}

// send an NTP request to the time server at the given address
unsigned long 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();
}