checking the validation for javascript

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Javascript

Dear All,

Please take a look at the following javascript:

function validate()
    {
            if(!checkDate( document.form1.txtFdate.value ) )
            {
                alert("Invalid Date");
                document.getElementById("txtFdate").focus();
                return false;
            }
            if(!checkDate( document.form1.txtTdate.value ) )
            {
                alert("Invalid Date");
                document.getElementById("txtTdate").focus();
                return false;
            }
            var dateString=document.form1.txtFdate.value;
            var Fdate = (dateString.substring(6,10))+(dateString.substring(3,5))+(dateString.substring(0,2));
            dateString=document.form1.txtTdate.value;
            var Tdate = (dateString.substring(6,10))+(dateString.substring(3,5))+(dateString.substring(0,2));
            if(parseInt(Tdate,10)<parseInt(Fdate,10))
            {
                alert("To Date Cannot Be Less Than From Date");
                document.getElementById("txtTdate").focus();
                return false;
            }
            if((parseInt(Tdate,10)-parseInt(Fdate,10))>6)
            {
                alert("Difference Between From and To Date Cannot Be More than 6 Days");
                document.getElementById("txtTdate").focus();
                return false;
            }

           }

Here Its working fine for the Individual month, but if I select 31st march to 1st April, it gives the same message saying "Difference Between From and To Date Cannot Be More than 6 Days".
What might be the problem in this? please help me.....