You can either have a genserver (blocking in init) which periodically polls some state in tzdata which allows it to check if the update is complete, or ignore debug level logs. The former might be problematic if genserver blocks for too long so that its supervisor fails.

First approach (might make the app fail to start and I haven’t actually tried it out):

defmodule TzdataWaiter do
  use GenServer

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(_opts) do
    wait_for_tzdata()    

    :ignore
  end

  defp wait_for_tzdata do
    case Tzdata.zone_list() do
      [] ->
        :timer.sleep(:timer.seconds(1))
        wait_for_tzdata()
      [_|_] -> 
        :ok
    end
  end
end

Second approach (if you just want to make the logs go away):

# if in iex
iex> Logger.configure(level: :info)

# if in config
config :logger, level: :info

Or you can completely purge the debug logs from Tzdata.ReleaseUpdater since it’s using Logger.debug macros (it wouldn’t have worked if it used Logger.log or :logger):

config :logger, 
  compile_time_purge_matching: [
    [module: Tzdata.ReleaseUpdater, level_lower_than: :info]
  ]

The first response by @ruslandoga lists several options, I think the first and last one look promising.

Thank you for the good ideas!

I tried putting Tzdata.ReleaseUpdater.check_if_time_to_update() at the top of my application start. This works for me because I can see that the tz has updated and these log messages appear before any log messages that my app sends.

I also tried Logger.put_module_level/2 and that seemed to work but then I worried that maybe I need to see logs sometimes.