default.exs

defmodule Example1 do
  @default_now DateTime.utc_now()

  def foo(now \\ @default_now) do
    IO.puts(now)
  end
end

defmodule Example2 do
  def foo(now \\ DateTime.utc_now()) do
    IO.puts(now)
  end
end

Example1.foo()
Example2.foo()
Process.sleep(10000)
Example1.foo()
Example2.foo()
Process.sleep(10000)
Example1.foo()
Example2.foo()

output

$ elixir test.exs
2021-07-14 15:01:48.062782Z
2021-07-14 15:01:48.088374Z
2021-07-14 15:01:48.062782Z
2021-07-14 15:01:58.088991Z
2021-07-14 15:01:48.062782Z
2021-07-14 15:02:08.089879Z