Trying manual epoch calculations

reinvented_wheel.sh

#!/usr/bin/env bash
#
#set -x
#
_date=$1
wheelv2 () {
	local d=$1
	local _year _month _day _hour _minute _second epoch today days_since_epoch seconds_since_epoch
	local pattern='[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]T[0-9][0-9]:[0-9][0-9]:[0-9][0-9]Z'
	if ! [[ "$d" =~ $pattern ]]; then
		printf '%s\n' 'Error: Unknown input date format' >&2
		exit 1
	fi
	_year=${d:0:4}
	_month=${d:5:2}
	_day=${d:8:2}
	_hour=${d:11:2}
	_minute=${d:14:2}
	_second=${d:17:2}
	if [[ "$_month" -gt 2 ]]; then
		_month=$((_month+1))
	else
		_month=$((_month+13))
		_year=$((_year-1))
	fi
	today=$(( (_year*365) + (_year/4) - (_year/100) + (_year/400) + (_month*306001/10000) + _day ))
	days_since_epoch=$(( today-719591 ))
	seconds_since_epoch=$(( (days_since_epoch*86400) + (_hour*3600) + (_minute*60) + _second ))
	printf '%s\n' "$seconds_since_epoch"
}
wheel_date=$(wheelv2 "$_date")
gnu_date=$(gdate -d "$_date" '+%s')
#
printf '%s: %s\n' \
	"Input date is" \
	"$_date"
#
printf '%s\n%s: %s\n%s: %s\n' \
	"Your results are as follows" \
	"GNU Date" "$gnu_date" \
	"Reinvented wheel" "$wheel_date"

$ ./reinvented_wheel.sh '2019-10-23T17:25:23Z'
Input date is: 2019-10-23T17:25:23Z
Your results are as follows
GNU Date: 1571851523
Reinvented wheel: 1571851523

Line 21 and 24 should be +1 and -1 not ++ and -- ... after that, appears to work.