File content as of revision 0:59aa41d04f51:

#include "mbed.h"

Serial pc(USBTX, USBRX);
DigitalIn btn(USER_BUTTON);
DigitalOut led(LED1);
Timer timer;
Timer debounce;

bool state = false;
bool prevState = false;
bool counting = false;

int main() {
    while(true) {
        state = btn;
        if(state != prevState) {
            debounce.start();
        }
        if(debounce.read_ms() > 20) {
            debounce.stop();
            debounce.reset();
            if(!state) {
                if(!counting) {
                    counting = true;
                    led = true;
                    timer.start();
                } else {
                    counting = false;
                    led = false;
                    timer.stop();
                    pc.printf("%i ms\n", timer.read_ms());
                    timer.reset();
                }
            }
        }
        prevState = state;
    }
}