#!/bin/ksh
# indentation added to make it easier to see the logic
day=1 # not needed, it is always set below to the last day of the previous month
# delete this duplicate line: #!/bin/ksh
# delete this duplicate line: day=1
month=1
year=2012
if [ $day -eq 1 ] # added a missing <semicolon> or <newline> here
then { # [ change to {
        if [[ $month -eq 1 || $month -eq 2 || $month -eq 4 || $month -eq 6 || $month -eq 8 || $month -eq 9 || $month -eq 11 ]]
        # <space> added after [ and before ] on above line
        # and [ and ] changed to [[ and ]], respectively, and
        # 31 day months changed from 12, 4, 6, 8, & 10 to 12, 1, 3, 5, 7, 8, & 10
        then prevmonth=31
        elif [[ $month -eq 5 || $month -eq 7 || $month -eq 10 || $month -eq 12 ]]
        # <space> added after [ and before ] on above line
        # and [ and ] changed to [[ and ]], respectively, and
        # 30 day months changed from 1, 3, 5, 7, 9, & 11 to 4, 6, 9, & 11
        then prevmonth=30
        elif [ $month -eq 3 ]  # this line could just be else
        # <space> added after [ and before ] on above line
        then {  if [[ $((year%100)) -eq 0 && $((year%400)) -eq 0 ]]
                # <space> added after [ and before ] on above line
                # and [ and ] changed to [[ and ]], respectively
                # and $((...)) added around both year%... calculations
                # and then [ changed to then {
                then prevmonth=29
                elif [[ $((year%100)) -ne 0 && $((year%4)) -eq 0 ]]
                # <space> added after [ and before ] on above line
                # and [ and ] changed to [[ and ]], respectively
                # and $((...)) added around both year%... calculations
                then prevmonth=29
                else prevmonth=28
                fi
        } # ] changed to }
        fi #this was missing
} # ] changed to }
fi #this was missing
if [[ $day -eq 1 && $month -eq 1 ]] # <space> added after if
# <space> added after [ and before ] on above line
# and [ and ] changed to [[ and ]], respectively
then    year=$(($year-1))
        # $(...) change to $((...))
fi
echo $prevmonth