unsigned long millis()
millis() returns the number of milliseconds that have elapsed since the board was powered up.
The number of milliseconds that have elapsed since powerup.
The maximum value of a 32-bit integer is 4294967295 and consequently this is the maximum number of milliseconds that can be counted. After this the counter resets to 0 and the counting starts again. This period is approximately 49.7 days.
millis() conforms to the Arduino 1.6.x API.
uint32_t ms = millis();