@Pablos,
setClockWithTimestamp e sincronizzi la libreria con i server NTP. 
Ho scassato le @@ per settimane per implementarla! XD
Paolo.
p.s. per i server NTP si può usare anche: it.pool.ntp.org (però bisogna abilitare il DNS su Arduino)
p.s.s. questo è il mio sketch compilato con la IDE 1.0:
#include <SPI.h>
#include <Ethernet.h>
#include <EthernetUdp.h>
#include <swRTC.h>
// Enter a MAC address for your controller below.
// Newer Ethernet shields have a MAC address printed on a sticker on the shield
byte mac[] = {
0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA };
unsigned int localPort = 8888; // local port to listen for UDP packets
IPAddress timeServer(192, 43, 244, 18); // time.nist.gov NTP server 192, 43, 244, 18
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
long interval = 3600000; // interval to control NTP Server
unsigned long previousMillis = 0; // will store last time LED was updated
// A UDP instance to let us send and receive packets over UDP
EthernetUDP Udp;
swRTC rtc;
void setup()
{
delay(2000);
unsigned long ts = 0;
Serial.begin(9600);
Serial.print("Serial Start! at ");
Serial.print(millis());
Serial.println(" microsecond.");
AvviaEthernet();
Serial.println("Ethernet started.");
while (ts==0)
{
Serial.println("Check Server NTP...");
ts=GetNTPtime();
if (ts==0) {
Serial.println("Wait 5 seconds...");
delay(5000);
}
}
rtc.stopRTC();
rtc.setClockWithTimestamp(ts, 1970);
rtc.startRTC();
Serial.println("swRTC synchronized.");
}
void loop()
{
int deltaT = 0;
unsigned long currentMillis = millis();
// stampa();
// delay(1000);
if(currentMillis - previousMillis > interval) {
// save the last time you blinked the LED
previousMillis = currentMillis;
// Controlla il server NTP
deltaT = GetNTPtime() - rtc.getTimestamp();
Serial.print("DeltaT : ");
Serial.println(deltaT);
}
// else{
// Serial.println("Millis: ");
// Serial.println(currentMillis);
// Serial.println(previousMillis);
// Serial.println(interval);
// Serial.println(" ");
// }
}
void AvviaEthernet()
{
// start Ethernet and UDP
if (Ethernet.begin(mac) == 0) {
Serial.println("Failed to configure Ethernet using DHCP.");
// no point in carrying on, so do nothing forevermore:
for(;;)
;
}
else
{
Serial.println("Ethernet configured using DHCP.");
Serial.print("Local IP: ");
Serial.println(Ethernet.localIP());
Serial.print("SubnetMask: ");
Serial.println(Ethernet.subnetMask());
Serial.print("Gateway: ");
Serial.println(Ethernet.gatewayIP());
Serial.print("DNS Server: ");
Serial.println(Ethernet.dnsServerIP());
}
Udp.begin(localPort);
}
// 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();
}
unsigned long GetNTPtime(){
sendNTPpacket(timeServer); // send an NTP packet to a time server
// wait to see if a reply is available
delay(1000);
if ( Udp.parsePacket() ) {
// 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;
Serial.print("Seconds since Jan 1 1900 = " );
Serial.println(secsSince1900);
// now convert NTP time into everyday time:
Serial.print("Unix time = ");
// Unix time starts on Jan 1 1970. In seconds, that's 2208988800:
const unsigned long seventyYears = 2208988800UL;
// subtract seventy years:
unsigned long epoch = secsSince1900 - seventyYears;
// print Unix time:
Serial.println(epoch);
// print the hour, minute and second:
Serial.print("The UTC time is "); // UTC is the time at Greenwich Meridian (GMT)
Serial.print((epoch % 86400L) / 3600); // print the hour (86400 equals secs per day)
Serial.print(':');
if ( ((epoch % 3600) / 60) < 10 ) {
// In the first 10 minutes of each hour, we'll want a leading '0'
Serial.print('0');
}
Serial.print((epoch % 3600) / 60); // print the minute (3600 equals secs per minute)
Serial.print(':');
if ( (epoch % 60) < 10 ) {
// In the first 10 seconds of each minute, we'll want a leading '0'
Serial.print('0');
}
Serial.println(epoch %60); // print the second
return epoch;
}
else {
Serial.println("NTP Server unavailable.");
return 0;
}
}
void stampa() {
Serial.print(rtc.getHours(), DEC);
Serial.print(":");
Serial.print(rtc.getMinutes(), DEC);
Serial.print(":");
Serial.print(rtc.getSeconds(), DEC);
Serial.print(" ");
Serial.print(rtc.getDay(), DEC);
Serial.print("/");
Serial.print(rtc.getMonth(), DEC);
Serial.print("/");
Serial.println(rtc.getYear(), DEC);
Serial.println(rtc.getTimestamp(), DEC);
}