datepicker2 not pick days before datepicker1

datepicker2 not pick days before datepicker1

Hello I have two datepicker: $("#dat1").datepicker and $("#dat2").datepicker
Both return correct date in textbox. I want to do, when I click dat2, I
can't choise days before dat1. First day if I choise is dat1+ 7 / next
week. How can I do?
example: textbox1 = 05/12/2013 textbox2 = not before 12/12/2013
My onselect: onSelect: function (date) { var d = new Date(date);});