Bash Date Usage For Naming

I am recording some scripting I used to create backup classification/retention naming. It can be simplified into one function easily but I kept it like this so I can copy and paste easier which function I need. Script is pretty self explanatory but basically it takes today's date and name my eventual backup file name based on some logic.

# cat test_class.sh 
HOSTNAME=$(hostname -s)
BACKUP_CLASSIFICATION="UNCLASSIFIED"

function retention_date() {
  MM=`date -d ${1} +%m`
  DD=`date -d ${1} +%d`
  DAY=`date -d ${1} +%u`

  if [ $DD == 01 ]
  then
     if [ $MM == 01 ]
     then
       BACKUP_CLASSIFICATION="YEARLY"
     else
       BACKUP_CLASSIFICATION="MONTHLY"
     fi
  else
    if (($DAY == 7)); then
     BACKUP_CLASSIFICATION="WEEKLY"
    else
     BACKUP_CLASSIFICATION="DAILY"
    fi
  fi

}

function retention_today() {
  MM=`date '+%m'`
  DD=`date '+%d'`
  DAY=`date +%u`
  
  if [ $DD == 01 ]
  then
     if [ $MM == 01 ]
     then
       BACKUP_CLASSIFICATION="YEARLY"
     else
       BACKUP_CLASSIFICATION="MONTHLY"
     fi
  else
    if (($DAY == 7)); then
     BACKUP_CLASSIFICATION="WEEKLY"
    else
     BACKUP_CLASSIFICATION="DAILY"
    fi
  fi

}

echo "TEST TODAY"
DATE=`date +%Y-%m-%d`
retention_today
echo $HOSTNAME-$BACKUP_CLASSIFICATION-$DATE
  
echo 
echo "TEST SPECIFIC DATES"
testD=(
 '2018-01-01'
 '2018-02-02'
 '2018-03-01'
 '2018-02-06'
 '2018-07-14'
 '2018-07-15'
)

for D in "${testD[@]}"
do
  DATE=`date -d ${D} +%Y-%m-%d`
  retention_date $D
  echo $HOSTNAME-$BACKUP_CLASSIFICATION-$DATE
done

Run and output.

# ./test_class.sh 
TEST TODAY
oci04-DAILY-2018-07-20

TEST SPECIFIC DATES
oci04-YEARLY-2018-01-01
oci04-DAILY-2018-02-02
oci04-MONTHLY-2018-03-01
oci04-DAILY-2018-02-06
oci04-DAILY-2018-07-14
oci04-WEEKLY-2018-07-15