пример вычисления количества дней до нового года:
var date1 = new Date();
var date2 = new Date("1/1/2015");
var diff = date2 - date1;
var milliseconds = diff;
var seconds = milliseconds / 1000;
var minutes = seconds / 60;
var hours = minutes / 60;
var days = hours / 24;
alert(days);
alert(Math.ceil(days));