Bash date manipulation

bash_date_manipulation.sh

#!/usr/bin/env bash

hour_trunc12 () {
    hr=$1
    echo "12 * ($hr / 12)" | bc
}

#  # 00 hours
#  echo $(hour_trunc12 0)  #> 0
#  echo $(hour_trunc12 11) #> 0
#  
#  # 12 hours
#  echo $(hour_trunc12 12) #> 12
#  echo $(hour_trunc12 23) #> 12

datetime=$(date --utc +%Y-%m-%dT%H:%M:%S+00:00)
echo $datetime

date_str=$(date --utc --date "${datetime}" +%Y-%m-%d)
echo $date_str

hr=$(date --utc --date "${datetime}" +%H)
hr=$(hour_trunc12 $hr)
echo $hr

date_hour=$(date --utc --date "${date_str} + ${hr} hours" +%Y-%m-%dT%H:%M:%S+00:00)
echo $date_hour