Creating a Tap Tempo (help needed)

you'll have to take the time between two presses to calculate the differnce in Milliseconds.
so the first thing is to distinguish between the 1st and the 2nd press.
use a variable to count them.

int numberOfButtonPresses = 0;
long int time_1;
long int time_2;

long int time_difference; //thats what your looking for

now in your main loop wait for the action.

if (buttonPressed && numberOfButtonPresses== 0 ) { //first action
//maybe reset millis ... resetMilis();
//store the "start time"
time_1 = millis();
numberOfButtonPresses++;
}

if (buttonPressed && numberOfButtonPresses== 1 ) { //SECOND action
//store the "end time"
time_2 = millis();
time_difference=time_2 - time_1;
numberOfButtonPresses=0; //reset button counter for the next time
}

HEllo,
Thank you very much, that's a big help.

Now i have an other question. I read that the millis() starts on startup. So on the first buttonPressed i need to reset the millis(), right?
But (sorry if this is a dump question) how do i reset the millis() and asign time_1 to the start time?
The same for time_2 and end time?

Is this also tru that you don't need an interrupt on overflow (because the millis() overflows on 9 hours .... )?

Thanks is advance.

regards
Ronald