Something

				function validedate()
				{
					dateaaaammjj = document.form1.DateNais.value;
					var dt=dateaaaammjj.split("-"),date=new Date(dt[0],dt[1]-1,dt[2]);
 					return date.getDate()==dt[2]&&date.getMonth()+1==dt[1]&&date.getFullYear()==dt[0]?date:false;
 				}