Min date and maximum date in angularjs.

Hi, 

I am learning angularjs.There is one application in which one of the form and it consist of start date and end date.

1) I mentioned the min date=start date  in end date.

when  I am editing those date when I select the end date bofore the start date at that time i can select any date which are befor the start date.I don't want like that I want end date always greater than or equal to start date.

 2)When I select start date before end date then min date is working finely.

Why this is happen and what should I do?