Just now saw your edit but it doesn’t change much. You can still make your mock respect timezone offsets.

Alternatively, I think you can just use a proper timezone database and instruct it to not do network requests.

tz has very configurable behaviour on when (or if?) to do automatic updates.

Here’s a link to the docs of tzdata that show you how to disable automatic updates.

Intuitively, I’d say just inspect Elixir’s source and see which timezone functions it uses when a timezone database is supplied?

Here’s what Elixir itself does when ultimately it gets down to calling a function from the timezone module: elixir/lib/elixir/lib/calendar/datetime.ex at 1a1eda52b46e37b0c77835e374c16ec64bb5c638 · elixir-lang/elixir · GitHub

Basically it calls its time_zone_periods_from_wall_datetime/2 function (first argument is datetime, second argument is a timezone).

Another example in Elixir itself is how such a timezone database can be faked. Here: elixir/lib/elixir/test/elixir/calendar/fakes.exs at 1a1eda52b46e37b0c77835e374c16ec64bb5c638 · elixir-lang/elixir · GitHub

The link is to the above function but the entire module should be looked at.

You can combine that knowledge with either mock or patch so you have a local offline fake timezone database.

Never tried what you need but I’d think that mock or patch will do the job.