textbox con mask per data

[Jordi75]

ciao,
Ho trovato questo script per far si che al focus di una textbox compaiano i c aratteri __/__/____ per inserire la data.
IL problema è che se inserisco una data (ad esempio 01/01/2000) e poi cancello alcuni caratteri, non me li fa più inserire. Potete aiutarmi a modificare il codice per correggere questo errore?

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

       // JScript File

       // Text Field add onfocus=javascript:DateControl('Textboxid')

       //it is automaticaly convert date Masked Date control

       //Created by M.Sathiskumar(16/04/2007)

       

       var c_txtid;

       var c_count=0;

       var Tchar;

       var bl_count=0;

       var astext="__/__/____"; //

       function DateControl(cnt_id)

       {

       c_txtid=cnt_id; 

       document.getElementById(cnt_id).style.textAlign="center"; 

       document.getElementById(cnt_id).setAttribute("maxlength",10)

       if(document.getElementById(cnt_id).value=="")

       document.getElementById(cnt_id).value="dd/mm/yyyy"; 

       document.getElementById(cnt_id).setAttribute("onblur",clear);

       document.getElementById(cnt_id).setAttribute("onkeydown",noNumbers);

       document.getElementById(cnt_id).maxlength="10"; 

       }

       function clear()

       {

       astext="__/__/____";//

       c_count=0;

       if (datechks()==1)

       {

       //alert("InValid Date");

       //document.write("InValid Date");

       document.getElementById(c_txtid).value="dd/mm/yyyy";

       document.getElementById(c_txtid).focus();

       }

       }

       

       

       function chk()

       {

       var ValidChars = "0123456789/_";

       var IsNumber=true;

       var Char; 

       var stext= document.getElementById(c_txtid).value;

       if (c_count==1) 

       { 

       //document.getElementById(c_txtid).value="__/__/____" 

       }

       

       

       for (i = 0; i < stext.length && IsNumber == true; i++) 

       { 

       Char = stext.charAt(i); 

       if (ValidChars.indexOf(Char) == -1) 

       {

       document.getElementById(c_txtid).value=stext.replace(Char,"")

       IsNumber = false; 

       }

       } 

       switch (Tchar){case '`': Tchar=0;break;case 'a': Tchar=1;break;case 'b': 
       Tchar=2;break;case 'c': Tchar=3;break;case 'd': Tchar=4;break;case 'e': 
       Tchar=5;break;case 'f': Tchar=6;break;case 'g': Tchar=7;break;case 'h': 
       Tchar=8;break;case 'i': Tchar=9;break;}

       switch (c_count)

       {

       case 1:

       if (Tchar<4)

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext; 

       }

       else

       {

       c_count--;

       }

       break;

       

       case 2: 

       if (Tchar<2 || stext.charAt(0)<3)

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       }

       else

       {

       c_count--;

       }

       break;

       

       case 3:

       if(Tchar<2) 

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       }

       else

       {

       c_count--;

       }

       break;

       

       case 4: 

       if (Tchar<3 || stext.charAt(3)=="0")

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       }

       else

       {

       c_count--;

       }

       break;

       

       case 5:

       if (Tchar==2 || Tchar==1 )

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       }

       else

       {

       c_count--;

       } 

       break;

       

       case 6:

       if (Tchar==9 || Tchar==0 )

       {

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       }

       else

       {

       c_count--;

       } 

       break;

       

       case 7:

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext;

       break;

       

       case 8:

       astext=astext.replace("_",Tchar)

       document.getElementById(c_txtid).value=astext; 

       break;

       

       case 9:

       c_count=0;

       astext="__/__/____";//

       document.getElementById(c_txtid).value="__/__/____" // 

       break;

       }

       

       

       return IsNumber ; 

       }

       

       function noNumbers()

       {

       var keynum

       var keychar

       var numcheck;

       if(window.event) // IE

       {

       keynum = event.keyCode

       }

       else if(event.which) // Netscape/Firefox/Opera

       {

       keynum = event.which

       }

       keychar = String.fromCharCode(keynum)

       Tchar=keychar;

       c_count++;

       numcheck = /\d/;

       //alert(keynum)

       if(keynum!=9 && ((keynum >47 && keynum <58) || (keynum >95 && keynum 
       <106) ) )

       {

       chk();

       }

       else

       return 0;

       return numcheck.test(keychar)

       

       }

       

       function datechks()

       { 

       var txtdate;

       var subtxt; 

       txtdate = document.getElementById(c_txtid).value;

       //alert(txtdate); //document.getElementById(txtid).value;

       txtdate=txtdate.replace(" ","") 

       if(txtdate.length > 0)

       {

       if(txtdate.length > 7 && txtdate.length < 11)

       { 

       subtxt =txtdate.substr(0,txtdate.indexOf("/"));

       txtdate=txtdate.substr(txtdate.indexOf("/")+1,txtdate.length); 

       if (subtxt >0 && subtxt < 32)

       { 

       subtxt =txtdate.substr(0,txtdate.indexOf("/"));

       txtdate=txtdate.substr(txtdate.indexOf("/")+1,txtdate.length); 

       if (subtxt >0 && subtxt < 13)

       { 

       if (txtdate.length > 3 )

       { 

       

       return 0;

       }

       } 

       }

       } 

       } 

       return 1;

       }

       

       

		</script>

........

<asp:TextBox id="TextBox1" style="Z-INDEX: 101; LEFT: 416px; POSITION: absolute; TOP: 128px" runat="server" onfocus="javascript:DateControl('TextBox1')" maxlength="10" Width="150"></asp:TextBox>