long day = 86400000; // 86400000 milliseconds in a day
long hour = 3600000; // 3600000 milliseconds in an hour
long minute = 60000; // 60000 milliseconds in a minute
long second =  1000; // 1000 milliseconds in a second

void setup(){
Serial.begin (57600);
}

void loop(){
 time();
 delay(1000);
}

void time(){
long timeNow = millis();

int days = timeNow / day ;                                //number of days
int hours = (timeNow % day) / hour;                       //the remainder from days division (in milliseconds) divided by hours, this gives the full hours
int minutes = ((timeNow % day) % hour) / minute ;         //and so on...
int seconds = (((timeNow % day) % hour) % minute) / second;

 // digital clock display of current time
 Serial.print(days,DEC);
 printDigits(hours);
 printDigits(minutes);
 printDigits(seconds);
 Serial.println();

}

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