Outputting if the 10 day period is finished - bash

Question

I want to write a bash script that determines the 10 day period (decade) has ended relative to the start date (in the format YYYY-MM-DD).
If the 10 day period is finished script has to output the 10 days period.
Im new in bash and has a lot syntax errors with code, help me pls.
#!/bin/bash
# GNU bash, version 4.3.46
CURRENT_DATE=$(date +%Y-%m-%d)
START_DATE=2019-01-01
IS_TODAY_DECADE_CALCULATION_DAY = (CURRENT_DATE - START_DATE) % 10
if [ $IS_TODAY_DECADE_CALCULATION_DAY -eq 0 ]
then
BEGIN_DATE = $("$CURRENT_DATE - 11 days" +%Y-%m-%d)"
END_DATE = $("$CURRENT_DATE - 1 day" +%Y-%m-%d)"
echo "Period is="$BEGIN_DATE":"$END_DATE"
else
echo "Decade is not finished."
fi

Answer 1

You should compare the unix time stamps. If the time stamp "now+10 days" is larger than the start date, the period is ended.
#! /bin/bash
DATE_OLD=$(date "+%F" -d "-11 days")
DATE_NOW=$(date "+%F")
TEST_DATE_NOW=$(date "+%s" -d ${DATE_NOW})
TEST_DATE_OLD=$(date "+%s" -d ${DATE_OLD})
DIVIDER=$(( (TEST_DATE_NOW - TEST_DATE_OLD) / (60*60*24) ))
REMAINING=$(( DIVIDER % 10 ))
echo "Days between ${DATE_OLD} and ${DATE_NOW} is $DIVIDER"
if [ ${DIVIDER} -gt 0 ]; then
echo "Date ${DATE_OLD} is in the past"
else
echo "Date ${DATE_OLD} is in the future"
fi
if [ $REMAINING -eq 0 ]; then
echo "Ten days period ended"
else
echo "Still in ten day period"
fi
exit 0;

Answer 2

The question implies that the code should identify each 10 day period starting on a specific START_DATE. Bash does not have date math - it can not calculate difference between dates (as expected by '(CURRENT_DATE - START_DATE)'). Two options
Convert date to seconds since Unix Epoch, and do the math on those values, OR
Use date utilities package, OR
using Python, awk, perl
Implementing #1 is simple. Notice few changes to assignments - in particular no spaces are allowed in assignments variable=expression, or let variable=expression
#! /bin/bash
CURRENT_DATE=$(date +%Y-%m-%d)
START_DATE=2019-01-01
# Instead of IS_TODAY_DECADE_CALCULATION_DAY = (CURRENT_DATE - START_DATE) % 10
SEC_IN_DAY=$((60*60*24))
let D1=$(date '+%s' -d "$CURRENT_DATE Z")/SEC_IN_DAY
let D2=$(date '+%s' -d "$START_DATE Z")/SEC_IN_DAY
let IS_TODAY_DECADE_CALCULATION_DAY=(CURRENT_DATE-START_DATE)%10
# Rest of script here
if [ $IS_TODAY_DECADE_CALCULATION_DAY -eq 0 ]
then
BEGIN_DATE=$(date -d "$CURRENT_DATE - 11 days" +%Y-%m-%d)
END_DATE=$(date -d "$CURRENT_DATE - 1 day" +%Y-%m-%d)
echo "Period is=$BEGIN_DATE:$END_DATE"
else
echo "Decade is not finished."
fi