Arduino_setRTC.ino

/*
 * RTC clock is set in response to serial port time message 
 * A Processing example sketch to set the time is included in the download
 * On Linux, you can use "date +T%s > /dev/ttyACM0" (UTC time zone)
 */

#include <TimeLib.h>
#include <DS3232RTC.h>
#include <Wire.h>

void setup() {
  Serial.begin(9600); 
  setSyncProvider(RTC.get);
  if (timeStatus() != timeSet)
    Serial.println("unable to sync with the RTC!");
  else
    Serial.println("synchronized with the RTC!");
}

void loop()
{
  if (Serial.available()) {
    time_t t = processSyncMessage();
    if (t != 0) {
      RTC.set(t);   // set the RTC and the system time to the received value
      setTime(t);          
    }
  }

  digitalClockDisplay();  
  delay(1000);
}

void digitalClockDisplay(){
  // digital clock display of the time
  Serial.print(hour());
  printDigits(minute());
  printDigits(second());
  Serial.print(" ");
  Serial.print(day());
  Serial.print(" ");
  Serial.print(month());
  Serial.print(" ");
  Serial.print(year()); 
  Serial.println(); 
}

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

/*  code to process time sync messages from the serial port   */
#define TIME_HEADER  "T"   // Header tag for serial time sync message

unsigned long processSyncMessage() {
  unsigned long pctime = 0L;
  const unsigned long DEFAULT_TIME = 1357041600; // Jan 1 2013 

  if(Serial.find(TIME_HEADER)) {
     pctime = Serial.parseInt();
     return pctime;
     if( pctime < DEFAULT_TIME) { // check the value is a valid time (greater than Jan 1 2013)
       pctime = 0L; // return 0 to indicate that the time is not valid
     }
  }
  return pctime;
}