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 dpkg-reconfigure tzdata.

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

change_debian_timezone

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