micros()

Returns the number of microseconds since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 70 minutes. On 16 MHz Arduino boards (e.g. Duemilanove and Nano), this function has a resolution of four microseconds (i.e. the value returned is always a multiple of four). On 8 MHz Arduino boards (e.g. the LilyPad), this function has a resolution of eight microseconds.

Note: there are 1,000 microseconds in a millisecond and 1,000,000 microseconds in a second.

None

Number of microseconds since the program started (unsigned long)

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

* millis() * delay() * delayMicroseconds()

Source: arduino.cc