Accept a time_zone argument to override system time?

Some of our users have set a time_zone preference and we want to respect that preference regardless of where their computer happens to be.

Start by locating the public API that currently derives local time from the system timezone and review its existing tests. Implementing the requested override should allow callers to provide a time_zone preference regardless of the computer's timezone, with tests showing both the default and overridden behavior.

Written by the indexing model from the issue text.