var timera = (4*60+31.5)*1000; //ms
var timerb = 10*60*1000; //ms
var timeofa, timeofb, timerbactive = false;


function updatetimera(time) {
    //update the timer a from the date object
}

function updatetimerb(time) {
    //update the timer b from the date object
}

function endtimera() {
    //probably something like
    var t = new Date();
    timeofa = t.getTime();
    updatetimera(t);
}
function endtimerb() {
    //set timer b's area to 'you can do the thing'
}

function timer() {
    var now = new Date();

    if (timeofa + timera < now.getTime()){
        updatetimera(now);
    }
    else {
        endtimera();
    }

    if (timerbactive && timeofb + timerb < now.getTime()){
        updatetimerb(now);
    }
    else {
        endtimerb();
    }
    setTimeout(timer, 500);
}

function buttona() {
    //respond to button a being pressed
    timeofa = (new Date()).getTime();
}

function buttonb() {
    //respond to button b being pressed
    timeofb = (new Date()).getTime();
    //end with:
    setTimeout(timer, 500);
}

$.ready(function() {
   //setup buttons and stuff, add onclick events
});