[Dilettante_Pro]

Например, с 8:30 до 20:30

var startTime      = (new Date()).setHours(8,30),
      finishTime    = (new Date()).setHours(20,30),
      currentTime = new Date();
if (currentTime >= startTime &&  currentTime <= finishTime) {
   alert('Рабочее время');
}