Check out example codes for "arduino millis()". It will help you in understanding the concepts better.

Code Example 1

/*Description
Returns the number of milliseconds passed since the Arduino board began running
the current program. This number will overflow (go back to zero), after
approximately 50 days.

Syntax
*/
time = millis();

Code Example 2

/*Description
Returns the number of milliseconds passed since the Arduino board began running the current program. This number will overflow (go back to zero), after approximately 50 days.

Syntax                                                                     */
time = millis()

/*
Returns
Number of milliseconds passed since the program started.
Return Data type: unsigned long.
*/

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