By setting applications: key you’re no longer using “application inference” [1], so you’d have to explicitly put all dependencies there if you want them started. The lack of :db_connection in the application list causes the crash.

It’s best not to set applications: key explicitly, and let inference do it’s work. Timex should probably show how to install it with and without application inference, if it doesn’t, I think a PR to update docs is likely to get accepted.

Worth mentioning that as of Elixir v1.8 you no longer need to use Timex to convert between timezones, this functionality is now available in stdlib using DateTime.shift_zone/3. You’d need a timezone database like the one provided by tzdata package [2]

There’re no built-in formatting options other than ISO 8601, so if you need to have a custom format you need to roll your own or bring in Timex. If formatting would be the only reason to bring Timex though, I’d reconsider that.

[1] elixir/CHANGELOG.md at v1.4 · elixir-lang/elixir · GitHub
[2] tzdata | Hex

You need to use Timex, it provides functions for TZ conversion and formatting.