Problemas com Calendario

Pessoal estou tentando adptar um calendario em uma pagina mas estou tendo problemas, por favor me ajudem

a pagina é essa

quando clico no link do proximo mes, funciona beleza

mas quando clico no link do mes anterior não funciona.

da o seguinte erro:

Erro de tempo de execução do Microsoft VBScript erro '800a000d'

Tipos incompatíveis

/agenda.asp, line 250

calendario

<%
res=Request.QueryString("Show")
whichmonth=Request.QueryString("CurMonth")
whichyear=Request.QueryString("CurYear")
whataction=Request.QueryString("Action")
%>
<%

dim nextmonth
dim acalandardays(42)
nextmonth = 0

dbcurrentdate = date()

if res="Yes" then
whichdate="01" & "/" & whichmonth & "/" & whichyear
if whataction="Next" then
dbcurrentdate=dateadd("m",1,cdate(whichdate))
elseif whataction="Prev" then
dbcurrentdate=dateadd("m",-1,cdate(whichdate))
else
dbcurrentdate=dbcurrentdate(whichdate)
end if
end if

Dim dbcurrentdate

dim ifirstweekday
ifirstweekday = datepart("W",dateserial(year(dbcurrentdate),month(dbcurrentdate),1))
dim idaysinmonth
idaysinmonth = datepart("d",dateserial(year(dbcurrentdate),month(dbcurrentdate)+1,1-1))
dim iloop
for iloop = 1 to idaysinmonth
acalandardays(iloop+ifirstweekday-1)=iloop
next

dim icolumns,irows
icolumns = 7
irows = 6-int((42-(ifirstweekday+idaysinmonth))/7)
%>

<table width=45% height=45% border=1 align =center cellpadding="0" cellspacing = "0" bordercolor="#FFCC00" bgcolor="#FFFF99">
<tr></tr>
	<th colspan=7 align="center" valign="middle" bgcolor=#FFCC00><a href="agenda.asp?Show=Yes&CurMonth=<%= left(curmonth,3)%>&CurYear=<%= curyear%>&Action= Prev" class="style38">Anterior</a>       
 <span class="style36">
 <%

curmonth=monthname(month(dbcurrentdate)) Response.Write curmonth curyear=year(dbcurrentdate) Response.Write " " & curyear %>

 </span>      <span class="style35"> 
 <a href="agenda.asp?Show=Yes&CurMonth=<%=left(curmonth,3)%>&CurYear=<%=curyear%>&Action=Next" class="style38">Próximo</a>

 <%
Response.Write	"<tr>"
Response.Write "<td align=center>"
Response.Write "<font color=#cc0000 >"

Response.Write "Domingo"

Response.Write "</font>"
Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Segunda"

Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Terça"

Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Quarta"

Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Quinta"

Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Sexta"

Response.Write "</td>"
Response.Write "<td color=#FFAF1C align=center>"

Response.Write "Sábado"

Response.Write "</td>"

Response.Write	"</tr>"

%>

</font>

<%
dim irowsloop ,icolumnsloop
startday=weekday("01" & "/" & monthname(month(dbcurrentdate)) & "/" & year(dbcurrentdate))
ctr=1

for irowsloop = 1 to irows
Response.Write "<tr>"
if ctr=1 then
colm = startday
else
colm=1
end if
if startday>1 then
diff=colm - 1
end if

for icolumnsloop=colm to icolumns
if acalandardays((irowsloop-1)*7 +icolumnsloop) >= 0 then
if diff>0 then
for p=1 to diff

Response.Write "<td valign= top align=right width=""14%"">"
Response.Write "</td>"

next
diff=0
end if
Response.Write "<td valign= top align=right width=""14%"">"
if icolumnsloop=1 then
Response.Write "<font color=#cc0000>"
end if
if month(dbcurrentdate)<month(Date()) then
if acalandardays((irowsloop-1)*7 + icolumnsloop)=day(Date()) and month(dbcurrentdate)=month(Date()) and year(dbcurrentdate)=year(Date()) then
Response.Write "<font>"
end if
Response.Write acalandardays((irowsloop-1)*7 + icolumnsloop)
elseif month(dbcurrentdate)=month(Date()) and acalandardays((irowsloop-1)*7 + icolumnsloop) < day(Date()) then
Response.Write acalandardays((irowsloop-1)*7 + icolumnsloop)
elseif month(dbcurrentdate)=month(Date()) and acalandardays((irowsloop-1)*7 + icolumnsloop) >= day(Date())then
Response.Write acalandardays((irowsloop-1)*7 + icolumnsloop)
elseif month(dbcurrentdate)>month(Date()) then
Response.Write acalandardays((irowsloop-1)*7 + icolumnsloop)
end if
Response.Write "</td>"
else
Response.Write "<td> </td>"
end if
ctr=2
next
Response.Write "<tr>"
colm=2
next
Response.Write "<br>"
%>
------------------------------------------------------------------------------------------------

if res="Yes" then

whichdate="01" & "/" & whichmonth & "/" & whichyear

if whataction="Next" then

dbcurrentdate=dateadd("m",1,cdate(whichdate))

elseif whataction="Prev" then

dbcurrentdate=dateadd("m",-1,cdate(whichdate))

else

dbcurrentdate=dbcurrentdate(whichdate) <<< linha 250

end if

end if

--------------------------------------------------------------------------------------------------

Desde já agradeço pela ajuda de todos.

valeu!