Hi,
Does your project run continuously 24hr/7days a week without the Arduino being turned on and off?
millis is declared as unsigned long, it will eventually rollover from 4,294,967,295 to 0.
This happens after about 50days.
Tom... 
Hi,
Does your project run continuously 24hr/7days a week without the Arduino being turned on and off?
millis is declared as unsigned long, it will eventually rollover from 4,294,967,295 to 0.
This happens after about 50days.
Tom...