This is easy!
create a countdown object and in the step event.
Initialize these vars, cur_time, min, sec;
var cur_time, min, sec, mili;
//Countdown code
cur_time = 10 //This is ten minutes not 10 seconds
cur_time = - min;
cur_time = +sec;
sec += 1;
if sec > 60 { min = -1
sec = 0;
};
now use the draw_text(x,y,string) function, the string can be anything but for this just use cur_time;
Hope that helps, never tested it though.