Its now working for me

<input type="date" id="tdate" ng-model="to_date" ng-change="GetCustomers(1)">
<input type="date" ng-model="from_date" ng-change="GetCustomers(1)">
<script type="text/javascript">
    var app = angular.module('MyApp', ['angularUtils.directives.dirPagination']);
    app.controller('MyController', function ($scope, $http) {
        $scope.Customers = [];
        $scope.PageIndex = 1;
        $scope.RecordCount = 0;
        $scope.PageSize = 20;
        $scope.SearchTerm = "";
        $scope.GetCustomers = function (index) {
            $scope.Customers = [];
            $http.post("StockLedgerwithAngular.aspx/GetCustomers",
            { 
                PageNo: index, 
                PageSize: $scope.PageSize, 
                searchTerm: $scope.SearchTerm, 
                selectedItem: $scope.selectedItem, 
                searchcat: $scope.searchcat, 
                to_date: Date.parse($scope.to_date), 
                from_date: Date.parse($scope.from_date)
            },
            { headers: { 'Content-Type': 'application/json'} })
            .success(function (response) {
                $scope.Customers = response.d.Customers;
                $scope.RecordCount = response.d.TotalRecords;
            });
        }
        $scope.GetCustomers($scope.PageIndex);
    });
</script>

<System.Web.Services.WebMethod()> _
Public Shared Function GetCustomers(ByVal PageNo As Integer, ByVal PageSize As Integer, ByVal searchTerm As String, ByVal selectedItem As String, ByVal searchcat As String, ByVal to_date As String, ByVal from_date As String) As CustomerData

End Function