If you need to change the timezone of a Debian system, your searches will probably tell you to use the tzselect utility. That will only change the timezone for the server temporarily. Use that command and then run this:

cat /etc/timezone

See how the timezone didn’t change? Left alone, the system will revert to the timezone stored in /etc/timezone at next reboot. Plus, tzselect does not offer the full compliment of possible timezones. I recently needed to change my server to GMT, for example, and tzselect does not include an option for it. Instead, Debian has a less-publicized tool that will solve both problems…

dpkg-reconfigure tzdata

Run that command and it will permanently change the timezone for the system, and includes options for all sort of obscure timezones. I wouldn’t consider GMT to be obscure, but maybe that’s just me. Took me forever to find this searching earlier today. I don’t know why this isn’t more widely noted, but there you go.