I run my unit tests with mix test --no-start, as I don’t want my application to start when I run my unit tests. However, I also want to test some code that uses Timex for a calculation. If I naively attempt to use Timex in my test, I get the following

    test "can get next instance of time in a trivial instance" do
      next_occurrence =
        Util.Time.get_next_occurrence_of_time(~D[2022-01-01], ~T[13:37:00], "America/New_York")

      assert next_occurrence == Timex.to_datetime({{2022, 1, 1}, {13, 37, 0}}, "America/New_York")
    end

  1) test get_next_occurrence_of_time can get next instance of time in a trivial instance (PillminderTest.Util.Time)
     test/pillminder/util/time_test.exs:8
     ** (ArgumentError) errors were found at the given arguments:
     
       * 1st argument: the table identifier does not refer to an existing ETS table
     
     code: assert next_occurrence == Timex.to_datetime({{2022, 1, 1}, {13, 37, 0}}, "America/New_York")
     stacktrace:
       (stdlib 3.17.2.1) :ets.lookup(:tzdata_current_release, :release_version)
       (tzdata 1.1.1) lib/tzdata/release_reader.ex:74: Tzdata.ReleaseReader.current_release_from_table/0
       (tzdata 1.1.1) lib/tzdata/release_reader.ex:17: Tzdata.ReleaseReader.simple_lookup/1
       (tzdata 1.1.1) lib/tzdata/release_reader.ex:9: Tzdata.ReleaseReader.zone_and_link_list/0
       (tzdata 1.1.1) lib/tzdata.ex:61: Tzdata.zone_exists?/1
       (timex 3.7.9) lib/timezone/timezone.ex:230: Timex.Timezone.name_of/1
       (timex 3.7.9) lib/timezone/timezone.ex:262: Timex.Timezone.get/2
       (timex 3.7.9) lib/timezone/timezone.ex:585: Timex.Timezone.convert/2
       (timex 3.7.9) lib/datetime/erlang.ex:46: Timex.Protocol.Tuple.to_datetime/2
       test/pillminder/util/time_test.exs:12: (test)

I can understand why this might be; tzdata likely needs to be started in order to read from this ETS table. I’m not sure the best way to fix this, because

1. If I remove --no-start, my application will start, which has side effects I don’t want to produce just by running my unit tests.
2. Even if I somehow prevent my application from running (is there a way?), but allow Timex to run tzdata, I’m still going to have side effects in my tests, Timex will need to update the timezone database before running my tests; I try to keep my unit tests free of network calls wherever I can.

How can I best deal with this? Is perhaps there some way I can “mock” the tzdata, or somehow pre-seed it for my unit tests?

Thanks!