Javascript code for validating date format

function  isValidDateFormat(date)  
{   

        var date = document.getElementById(date).value; 
         
	var m = date.substring(3,5);
	var d = date.substring(0,2);
	var y = date.substring(6,10);

	if((date.length > 10) 
          ||((date.charAt(2)!="-") 
          &&(date.charAt(2)!="/"))
          ||((date.charAt(5)!="-")
          &&(date.charAt(5)!="/"))
          ||(d>31) ||(m>12) ||(y<1900))
	{ 

	 	  return false;

	}
	else
        {
   
		 var m = date.substring(3,5);
		 var d = date.substring(0,2);
		 var y = date.substring(6,10);
	
		if (d==0 ||m==0 ||y==0 )
		{
	
                	 return false;
		 
		}
		
		if(m==01||m==03||m==05||m==07||m==08||m==10||m==12)
		{

			var dmax = 31;
	        
		}
		else if (m==04||m==06||m==09||m==11)
		{

		  var dmax = 30;
		  
		}
		else
		{

			if((y%400==0) || (y%400==0 && y%100!=0)) 
                        {

			     dmax = 29;

			}

		    else dmax = 28;

		}
		 
        if (d>dmax)

	    return false;
				
       }
	
       return true;

}