Proper unit test mocking for timezone aware DateTime objects

Questions / Help
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1

Does anyone have any experience with unit testing modules that need to be tested using DateTime objects with different timezones without having a real timezone database?

I am having trouble understanding how I would mock the timezone database in a way that would allow using methods such as DateTime.new!(Date.t, Time.t, Calendar.time_zone, Calendar.time_zone_database) without failing since the timezone database is not available.

2

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

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3

I know mock, but I don’t know how to mock a timezone database with it.

I don’t want to mock the functions I am calling, because I want to assert that the timezone conversions are taking place in the modules I am testing.

4

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/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/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.

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5

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.

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6

Thank you! I’ll take a look and share the results

7

Using past datetimes on non disputed regions should give you a rather stable timezone definition.

If you still need more control then you can use Mox with the time zone database behaviour and mock that.