Time

micros()

Returns the number of microseconds since the device booted.

unsigned long time = micros();

// EXAMPLE USAGE
SerialLogHandler logHandler;

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

It overflows at the maximum 32-bit unsigned long value.