AIX

#!/bin/ksh
#
# showlast.ksh
#
# show users that have logged X months ago

# define months
set -A months Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

# check command line for number of months
if [ "$#" -ne 1 ]
then
    echo "Usage: ${0##*/} <num of months>"
    exit 1
fi

# set the current month
cur_month=$(date '+%m')

# store the month from the command line
month_num=$1

# remove the zero padding if present
srch_month=$(echo $month_num|sed 's/0//')

# subtract the number of months from the current
# month
get_month=$(expr $cur_month - $srch_month)

# get month name from array by subtracting one from
# the number of months to match the correct array indice
month_index=$(expr $get_month - 1)
month_name=${months[$month_index]}

# call the last command and display the given number of
# months ago from current month
clear
echo "Logins From $month_num Months Ago"
echo
last | egrep $month_name | awk '!_[$1]++' | egrep -v 'reboot|shutdown' | sort -k1
echo

# done
exit 0

Running the script:
./showlast.ksh 2

Logins From 2 Months Ago

user1      pts/1        x.x.x.x                May 31 15:08 - 15:33  (00:25)
user2      rsh852070    x.x.x.x                May 31 15:17 - 15:17  (00:00)
user3      pts/1        x.x.x.x                May 31 14:54 - 15:07  (00:12)
user4      pts/0        x.x.x.x        	       May 21 17:22 - 17:49  (00:26)

#!/bin/ksh
#
# showlast
#
# show users that have logged X months ago

# define months
set -A months Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

# get list of users
tmpfile="/tmp/showlast.tmp"

# check command line for number of months
if [ "$#" -ne 1 ]
then
    echo "Usage: ${0##*/} <num of months>"
    exit 1
fi

# set the current month
cur_month=$(date '+%m')

# store the month from the command line
month_num=$1

# remove the zero padding if present
srch_month=$(echo $month_num|sed 's/0//')

# subtract the number of months from the current
# month
get_month=$(expr $cur_month - $srch_month)

# get month name from array by subtracting one from
# the number of months to match the correct array indice
month_index=$(expr $get_month - 1)
month_name=${months[$month_index]}

# call the last command and display the given number of
# months ago from current month
clear
echo "Logins From $month_num Months Ago"
echo
last | egrep $month_name | awk '!_[$1]++' | egrep -v 'reboot|shutdown' | sort -k1
echo

# dump our users that have logged in within the given number
# of months to the temp file 
last | egrep $month_name | awk '!_[$1]++' | egrep -v 'reboot|shutdown' | sort -k1 > $tmpfile

# dump a list the user accounts (minus the system accounts) to an array
set -A userlist $(awk -F: '$1 ~ /bin|daemon|sys|adm|uucp|lp|lpd|invscout|snapp|nuucp|sshd|ipsec|radius/{next}{print $1}' /etc/passwd)

# store number of elements in userlist
num_of_users=${#userlist[@]}

# loop through the lastusers array and print users that have not 
# logged in during the given number of months
echo 
echo "Non-Active Users From $month_num Months Ago"
echo
count=0
usercount=0
while [ $count -lt $num_of_users ]
do
    match=$(grep -c ${userlist[$count]} $tmpfile)
    if [ $match -eq 0 ]
    then
        echo ${userlist[$count]}
    fi
    ((count=$count+1))
done
echo

# done
rm -f $tmpfile
exit 0