Bash TimeZone Converter - MacOS

tz.sh

#!/bin/bash
TZ='CST6CDT' date
TZ='EST' date
TZ='UTC' date
date

while [[ $# -gt 0 ]] && [[ "$1" == "--"* ]] ;
do
    opt="$1";
    shift;              #expose next argument
    case "$opt" in
        "--" ) break 2;;
        "--cst2ist" )
           CST_TIME="$1"; shift;;
        "--cst2ist="* )     # alternate format: --first=date
           CST_TIME="${opt#*=}";;
        "--ist2cst" )
           IST_TIME="$1"; shift;;
        "--ist2cst="* )
           IST_TIME="${opt#*=}";;
        "--optional" )
           OPTIONAL="$optional_default";;     #set to some default value
        "--optional=*" )
           OPTIONAL="${opt#*=}";;             #take argument
        *) echo >&2 "Invalid option: $@"; exit 1;;
   esac
done

echo ''

if [ -n "$CST_TIME" ]
then
  echo "$CST_TIME CST in IST is"
  TZ="Asia/Calcutta" gdate --date 'TZ="CST6CDT"'$CST_TIME
fi

echo ''

if [ -n "$IST_TIME" ]
then
  echo "$IST_TIME IST in CST is"
  TZ="CST6CDT" gdate --date 'TZ="Asia/Calcutta"'$IST_TIME
fi

tz --cst2ist 03:30 --ist2cst 15:00

Thu Feb 11 01:33:26 CST 2021
Thu Feb 11 02:33:26 EST 2021
Thu Feb 11 07:33:26 UTC 2021
Thu Feb 11 13:03:26 IST 2021

03:30 CST in IST is
Thu Feb 11 15:00:00 IST 2021

15:00 IST in CST is
Thu Feb 11 03:30:00 CST 2021