<asp:HiddenField ID="hdnCurrentDate"  runat="server" />

hdnCurrentDate.Value = DateTime.Now.ToString("dd-MM-yyyy");

<asp:CustomValidator ID="custDateOfBirth"  runat="server" 	ControlToValidate="txtBirthDate" ClientValidationFunction="CheckDate" ErrorMessage="Enter less than the current date" CssClass="ClsRedtext"  SetFocusOnError="true"></asp:CustomValidator>

<script>
function CheckDate(oSrc,args)
{
//The below two ids are view source of the aspx file 
//of the respective controls one is Entered date and the other is hidden field date 
//('ctl00_ContentPlaceHolder1_txtBirthDate')
//('ctl00_ContentPlaceHolder1_hdnCurrentDate')

var dateOfBirthClientID =  document.getElementById('ctl00_ContentPlaceHolder1_txtBirthDate');    
var systemDateClientID =  document.getElementById('ctl00_ContentPlaceHolder1_hdnCurrentDate');
       
       var dateOfBirth = dateOfBirthClientID.value;
       var systemDate = systemDateClientID.value;
       
       var ArrDob =   dateOfBirth.split("-");
             
       var DobMonth = ArrDob[0];       
       var DobDate   = ArrDob[1];
       var DobYear   = ArrDob[2];
   
       var ArrSysDate =   systemDate.split("-");
              
       var SysDate   = ArrSysDate[0];
       var SysMonth = ArrSysDate[1];
       var SysYear   = ArrSysDate[2];
       
       //alert("DobYear="+DobYear+"SysYear ="+SysYear +"DobMonth="+DobMonth+"SysMonth="+SysMonth+"DobDate="+DobDate+"SysDate="+SysDate);
       
       if(DobYear > SysYear)
       {
               args.IsValid = false; 
               return; 
       }
       else if((DobYear == SysYear) && (DobMonth > SysMonth))
       {
               args.IsValid = false;  
               return; 
       }
       
        else if((DobYear == SysYear) && (DobMonth == SysMonth) && (DobDate > SysDate))
       {
               args.IsValid = false;  
               return; 
       }
        args.IsValid = true;
   }
</script>