Time

millis()

Returns the number of milliseconds since the device began running the current program. This number will overflow (go back to zero), after approximately 49 days.

unsigned long time = millis();

// EXAMPLE USAGE
SerialLogHandler logHandler;

void setup()
{
}

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

Note: The return value for millis is an unsigned long, errors may be generated if a programmer tries to do math with other data types such as ints.

Instead of using millis(), you can instead use System.millis() that returns a 64-bit value that will not roll over to 0 during the life of the device.