cat script.ksh
START_DATE=$1
END_DATE=$2;
if [[ -n $1 && -n $2 ]]
then
        awk -vstart_date="$START_DATE" -vend_date="$END_DATE" 'BEGIN{
                                                                        num=split("JAN FEB MAR APR MAY JUN JULY AUG SEPT OCT NOV DEC", array," ");
                                                                        for(i=1;i<=num;i++){
                                                                                                Q[array[i]]=i
                                                                                           };
                                                                        split(start_date, A,"-");
                                                                        split(end_date, B,"-");
        if(A[3] < B[3]){VAL_YEAR="START date year is less than END date year."}
        if(A[3] > B[3]){VAL_YEAR="START date year is greater than END date year."}
        if(A[3] == B[3]){VAL_YEAR="START date year is same as END date year."}
        if(Q[toupper(A[2])] < Q[toupper(B[2])]){VAL_MONTH="START date month is before than END date month."}
        if(Q[toupper(A[2])] > Q[toupper(B[2])]){VAL_MONTH="START date month is after than END date month."}
        if(Q[toupper(A[2])] == Q[toupper(B[2])]){VAL_MONTH="START date month is same as END date month."}
        if(A[1] < B[1]){VAL_DATE="START date is before than END date."}
        if(A[1] > B[1]){VAL_DATE="START date is after than END date."}
        if(A[1] == B[1]){VAL_DATE="START date is same as END date."}
        if(A[3] <= B[3] && Q[toupper(A[2])] <= Q[toupper(B[2])]){
        if((A[3] == B[3] && Q[toupper(A[2])] == Q[toupper(B[2])] && A[1] < B[1]) || (A[3] == B[3] && Q[toupper(A[2])] < Q[toupper(B[2])]) || (A[3] < B[3] && Q[toupper(A[2])] == Q[toupper(B[2])] && A[1] < B[1]) || (A[3] < B[3] && Q[toupper(A[2])] < Q[toupper(B[2])])){
                                print "start_date " start_date " seems to be less than " end_date " end_date."
                           }
        else               {
                                print "Date is NOT in proper format because of following values:"
                                print VAL_YEAR ORS VAL_MONTH ORS VAL_DATE
                           }
                           }
        else               {
                                print "Date is NOT in proper format because of following values:"
                                print VAL_YEAR ORS VAL_MONTH ORS VAL_DATE
                           }
                           }                                  '
else
        echo "One of the argument is seems to be missing while running the script."
fi

./script.ksh 29-MAY-2015 21-May-2015
Date is NOT in proper format because of following values:
START date year is same as END date year.
START date month is same as END date month.
START date is after than END date.