You seem to be making a lot of hard work for yourself when there are built in functions that will do all of the date calculation work for you. Look into using the DATEADD function, maybe coupled with DATEPART and then you just need to pass in valid dates, plus or minus any number of units of time measurement to get the required results.
A quick example:
<%
dim strMonth, strDay, strYear, strDate
strMonth = "2"
strDay = "23"
strYear = "2004"
strDate = strMonth & "/" & strDay & "/" & strYear
response.write dateAdd("d", 7, strDate) ' will add 7 days to the supplied date and change days, months, years as required
%>