How to validate multiple textbox with popup calendar?

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Javascript

The below code is used to validate start and end date(two textbox and date is selected from calendar)
when a button is clicked.The problem is i have 8 more start and end date to validate.so can anyone tell me a common code to validate all the start and end date

function validated() {
       var cal1=document.getElementById('<%=EndDateTwo.ClientId%>').value;
       var cal2= document.getElementById('<%=StartDateTwo.ClientId%>').value;
             
                
                if (cal1 == '' || cal2 == '') {
                    alert("Start Date and End Date  can not bleft blank.");
                    return false;
                }
                var dt1 = Date.parse(cal1);
                var dt2 = Date.parse(cal2);
                if (dt1 <= dt2) {
                    alert("End Date must occur after the Start date ");
                    return false;
                }
                return true;

<asp:TextBox ID="StartDateTwo" runat="server"></asp:TextBox>
<asp:HyperLink ID="hypCalenStartTwo" runat="server" ImageUrl="~/images/ico-cal.gif"></asp:HyperLink>
<ajax:CalendarExtender ID="StartDatePickerTwo"  runat="server" PopupButtonID="hypCalenStartTwo"
TargetControlID="StartDateTwo" SelectedDate='<%# Datetime.Today() %>' Format="MM/dd/yyyy">
</ajax:CalendarExtender>

<asp:TextBox ID="EndDateTwo" runat="server" ></asp:TextBox>
<asp:HyperLink ID="HypCalenEndTwo" runat="server" ImageUrl="~/images/ico-cal.gif"></asp:HyperLink>
 <ajax:CalendarExtender ID="EndDatePickertwo"  runat="server" PopupButtonID="hypCalenEndTwo"
 TargetControlID="EndDateTwo" SelectedDate="<%# Datetime.Today() %>" Format="MM/dd/yyyy">
</ajax:CalendarExtender>