Get Previous Date in Bash

getPreviousDate.sh

#!/bin/sh

year=`date +%Y`
month=`date +%m`
day=`date +%d`
isLeap=0

if [ $((year%400)) -eq 0 ]; then
  isLeap=1
elif [ $((year%100)) -eq 0 ]; then
  isLeap=0
elif [ $((year%4)) -eq 0 ]; then
  isLeap=1
fi

if [ $day -eq 01 ]; then
  if [ $month -eq 03 ] && [ $isLeap -eq 1 ]; then
    prevDay=29
  elif [ $month -eq 03 ] && [ $isLeap -eq 0 ]; then
    prevDay=28
  elif [ $month -eq 01 ] || [ $month -eq 02 ] || [ $month -eq 04 ] || [ $month -eq 06 ] || [ $month -eq 08 ] ||  [ $month -eq 09 ] || [ $month -eq 11 ]; then
    prevDay=31
  elif [ $month -eq 05 ] || [ $month -eq 07 ] || [ $month -eq 04 ] || [ $month -eq 10 ] || [ $month -eq 12 ]; then
    prevDay=30
  fi

  if [ $month -eq 01 ]; then
    prevMonth=12
    prevYear=$((year-1))
  else
    prevMonth=$(echo $month | sed 's/^0//')
    prevMonth=$((prevMonth-1))
    if [ $prevMonth -lt 10 ]; then
      prevMonth=0$prevMonth
    fi
    prevYear=$year
  fi
else
  prevDay=$(echo $day | sed 's/^0//')
  prevDay=$((prevDay-1))
  if [ $prevDay -lt 10 ]; then
    prevDay=0$prevDay
  fi
  prevMonth=$month
  prevYear=$year
fi

echo "Previous Date is: $prevMonth/$prevDay/$prevYear"
echo "Today's Date is: $month/$day/$year"