Javascript validate date time [safest way]

<script type="text/javascript" language="javascript">

 

        function isLeapYear() {

            return (year % 4 != 0 ? false :

                ( year % 100 != 0? true:

                ( year % 1000 != 0? false : true)));

        }

 

        function isValidDate(date_string) {

            var days = [0,31,28,31,30,31,30,31,31,30,31,30,31];

            var year, month, day, date_parts = null;

            var rtrn = false;

            var format = 'mm/dd/yyyy';

            var decisionTree = {

                'm/d/y':{

                    're':/^(\d{1,2})[./-](\d{1,2})[./-](\d{2}|\d{4})$/,

                    'month': 1,'day': 2, year: 3

                },

                'mm/dd/yy':{

                    're':/^(\d{1,2})[./-](\d{1,2})[./-](\d{2})$/,

                    'month': 1,'day': 2, year: 3

                },

                'mm/dd/yyyy':{

                    're':/^(\d{1,2})[./-](\d{1,2})[./-](\d{4})$/,

                    'month': 1,'day': 2, year: 3

                },

                'y/m/d':{

                    're':/^(\d{2}|\d{4})[./-](\d{1,2})[./-](\d{1,2})$/,

                    'month': 2,'day': 3, year: 1

                },

                'yy/mm/dd':{

                    're':/^(\d{1,2})[./-](\d{1,2})[./-](\d{1,2})$/,

                    'month': 2,'day': 3, year: 1

                },

                'yyyy/mm/dd':{

                    're':/^(\d{4})[./-](\d{1,2})[./-](\d{1,2})$/,

                    'month': 2,'day': 3, year: 1

                }

            };

            var test = decisionTree[format];

            if (test) {

                date_parts = date_string.match(test.re);

                if (date_parts) {

                    year = date_parts[test.year] * 1;

                    month = date_parts[test.month] * 1;

                    day = date_parts[test.day] * 1;

 

                    test = (month == 2 &&

                            isLeapYear() &&

                            29 ||

                            days[month] || 0);

 

                    rtrn = 1 <= day && day <= test;

                }

            }

            return rtrn;

        }

    </script>