millis()

Returns the number of milliseconds since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 50 days.

None

Number of milliseconds since the program started (unsigned long)

unsigned long time;
 
void setup(){
  Serial.begin(9600);
}
void loop(){
  Serial.print("Time: ");
  time = millis();
  //prints time since program started
  Serial.println(time);
  // wait a second so as not to send massive amounts of data
  delay(1000);
}

Note that the parameter for millis is an unsigned long, errors may be generated if a programmer tries to do math with other datatypes such as ints.

* micros() * delay() * delayMicroseconds()

Source: arduino.cc