js否返回false了按钮还执行PostBackUrl怎么办

js否返回false了按钮还执行PostBackUrl怎么办
<script language='javascript' type="text/javascript">
        function checkdate() {
            var bdate = document.getElementById("BeginDate").value;毕业论文 
            var edate = document.getElementById("EndDate").value;
            if (bdate.length == 0 || edate.length == 0) {
                alert("请选择起始时间或截止时间！");
                return false;
            }

            else{return true;}
        }
    </script>

【上面是JS下面是按钮代码】

<asp:Button ID="CheckBtn" runat="server"   Text="查询" PostBackUrl="~/result.aspx" OnClientClick="checkdate()" />

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当条件(bdate.length == 0 || edate.length == 0)符合时，弹出了警告窗，可是点确定后页面还是继续跳到result.aspx页面，求解决方法！！！！我希望是点掉警告窗后不执行跳转页面啊！条件不符合的时候再跳转

OnClientClick="return checkdate()" 
缺少一个return

1 <asp:Button ID="CheckBtn" runat="server"   Text="查询" PostBackUrl="~/result.aspx" OnClientClick="return checkdate()" />