$ mix new timeconverter --sup
* creating README.md
* creating .gitignore
* creating mix.exs
* creating config
* creating config/config.exs
* creating lib
* creating lib/timeconverter.ex
* creating lib/timeconverter/application.ex
* creating test
* creating test/test_helper.exs
* creating test/timeconverter_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

    cd timeconverter
    mix test

Run "mix help" for more commands.

defmodule Timeconverter.Mixfile do
  ...
  defp deps do
    [
      {:cowboy, "~> 1.0"},
      {:plug, "~> 1.0"}
    ]
  end
 end

defmodule Timeconverter.Router do
  use Plug.Router
  
  plug :match
  plug :dispatch

  get "/" do
    conn
    |> send_resp(200, "wow!")
  end

  match _ do
    conn
    |> send_resp(200, "woooow!")
  end
  
  def start_link do
    {:ok, _} = Plug.Adapters.Cowboy.http(Timeconverter.Router, [])
  end
end

defmodule Timeconverter.Application do
  ...
  def start(_type, _args) do
    ...
    children = [
      # Starts a worker by calling: Timeconverter.Worker.start_link(arg1, arg2, arg3)
      # worker(Timeconverter.Worker, [arg1, arg2, arg3]),
      worker(Timeconverter.Router, [])
    ]
    ...
  end
end

defmodule Timeconverter do
  @moduledoc """
  Documentation for Timeconverter.
  """
  
  def convert_datetime(%{"format" => format, "time" => time}) do
    case format do
      "to_unix"    -> iso8601_to_unix(time)
      "to_iso8601" -> unix_to_iso8601(time)
      _            -> {:error, ~s{Format should be either "to_unix" or "to_iso8601"}}
    end
  end
  def convert_datetime(%{}), do: {:error, ~s{time and format parameters should be specified. Format should be either "to_unix" or "to_iso8601", and time should be in either of those formats.} }

  defp iso8601_to_unix(time) do
    valid_iso8601 = ~r/^(?:[0-9]\d{3}-(?:(?:0[1-9]|1[0-2])-(?:0[1-9]|1\d|2[0-8])|(?:0[13-9]|1[0-2])-(?:29|30)|(?:0[13578]|1[02])-31)|(?:[1-9]\d(?:0[48]|[2468][048]|[13579][26])|(?:[2468][048]|[13579][26])00)-02-29)T(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:Z|[+-][01]\d:[0-5]\d)$/
    case Regex.match?(valid_iso8601, time) do
      true -> {:ok, time |> DateTime.from_iso8601() |> elem(1) |> DateTime.to_unix()}
      _    -> {:error, ~s{Invalid format for iso8601 input. Should be like "2015-01-17T21:23:02+00:00"}}
    end
  end

  defp unix_to_iso8601(time) do
    valid_unix = ~r/^[-+]?\d+$/
    case Regex.match?(valid_unix, time) do
      true -> {:ok, time |> String.to_integer() |> DateTime.from_unix!() |> DateTime.to_iso8601()}
      _    -> {:error, ~s{Invalid format for unix input. Should be like "1464096368"}}
    end
  end
end

defmodule Timeconverter.Router do
  ...
  get "/" do
    conn = Plug.Conn.fetch_query_params(conn)
    result = inspect(Timeconverter.convert_datetime(conn.params))
    conn
    |> send_resp(200, result)
  end

  match _ do
    conn
    |> send_resp(404, "This is not the page you are looking for")
  end
  ...
end

# Erlang version
erlang_version=19.2

# Elixir version
elixir_version=1.4.1

defmodule Timeconverter.Router do
  ...
  def start_link do
    {:ok, _} = Plug.Adapters.Cowboy.http(Timeconverter.Router, [], port: get_port())
  end
  ...
  defp get_port() do
    port_env_variable = System.get_env("PORT")
    if is_nil(port_env_variable) do
      4000
    else
      String.to_integer(port_env_variable)
    end
  end
end