Create a regular 10ms CTC timer interrupt.

The user sets the number of seconds.

Every 100'th IRQ, you decrement the second counter if still running.

e.g. ISR

if (++ms10 >= 100) {   // a whole second
    ms10 = 0;
    if (seconds) seconds--;
    else LED = off;
}

and setting the second count. You could disable IRQs while altering these variables, but not really necessary since ms10 is a char variable.

ms10 = 0;              // reset the jiffy count
seconds = 15;          // new countdown
LED = on;

David.