To set the timezone on a Server Core 2008 R2/2012, or any other version if you fancy CLI, use the following:

TZUTIL /G to list the current timezone
TZUTIL /L to list all available timezone settings
TZUTIL /S “Central Standard Time” to set your timezone to CST as example

Also useful is the w32tm tool to change settings around the Windows Time Service, such as setting up a manual NTP peer or forcing a time resync.