When you want change timezone in debian linux distors like debian stretch or ubuntu, you can use one very simple command in your command line
I have a timezone in United States and I want change this to Europe/Prague. I check my date now:
]$ date Tue Mar 20 11:27:20 EDT 2018
After you type command
dpkg-reconfigure tzdata you can see a graphical choice to your country.
]$ dpkg-reconfigure tzdata
Successfull timezone change write to your shell:
Current default time zone: 'Europe/Prague' Local time is now: Tue Mar 20 16:30:29 CET 2018. Universal Time is now: Tue Mar 20 15:30:29 UTC 2018.
After this my date is now:
]$ date Tue Mar 20 16:30:50 CET 2018
I can check my timezone in this file:
]$ cat /etc/timezone Europe/Prague