var now = new Date();
var millisTill10 = new Date(now.getFullYear(), now.getMonth(), now.getDate(), 10, 0, 0, 0) - now;
if (millisTill10 < 0) {
millisTill10 += 86400000; // it's after 10am, try 10am tomorrow.
}
setTimeout(function(){alert("It's 10am!")}, millisTill10);