increment month of date

I'm writing a shell script in unix about incrementing date. I want to increment the month of the current date whose format is `date +%b%Y`. So today the current date is May2004, i want the output to be Jun2004 and so if the current date is Dec2004, the output will be Jan2005. Need your advise asap. Thnx in advance.

#!/bin/ksh

getNextMonth() {
aMonth=$(echo $1 | cut -c1-3); yr=$(echo $1 | cut -c4-)
months=$(cal $yr | grep "[A-Z][a-z][a-z]")

i=1; n=$(for mon in $months; do [[ "$mon" = "$aMonth" ]] && echo $i || (( i += 1 )); done)
[[ "$n" -eq 12 ]] && n=1 && (( yr += 1 )) || (( n += 1 ))
echo "$(echo $months | cut -d" " -f$n)$yr"
}
# test next month function ----------------------------------------------
for mmmdd in May2004 Dec2004 Jan2004 Mar2004 Aug2004 Dec2009
do
echo "in = $mmmdd \nout = $(getNextMonth $mmmdd)"
done

Thanks so much mcq1 for the script, it's now working. 'Til my next script problem ok, tc.

how to increment month in asp can anybody help me???????
i m in big trouble.. plz!!!!!!!!!!

mn=month(fnsdt)
mn1=mn+1
or
directly i have done that increment but still it is not working,

bar="1"&"/"&Monthname(month(fnsdt)+1)&"/"&year(fnsdt)