diff --git a/lib/chronoscope/nts/client.ex b/lib/chronoscope/nts/client.ex
index 00ac080..cf52078 100644
--- a/lib/chronoscope/nts/client.ex
+++ b/lib/chronoscope/nts/client.ex
@@ -12,7 +12,7 @@ defmodule Chronoscope.NTS.Client do
 
   @impl true
   def init(%{host: host, port: port}) do
-    now = DateTime.utc_now()
+    now = utc_now()
 
     {:ok,
      %{
@@ -36,7 +36,7 @@ defmodule Chronoscope.NTS.Client do
   end
 
   defp update_state(state) do
-    now = DateTime.utc_now()
+    now = utc_now()
 
     if interval_surpassed?(now, state.last_key_establishment) do
       Map.merge(state, current_data(state, now))
@@ -63,4 +63,12 @@ defmodule Chronoscope.NTS.Client do
   defp interval_surpassed?(now, last_key_establishment) do
     DateTime.diff(now, last_key_establishment, :second) >= @interval_in_seconds
   end
+
+  defp utc_now() do
+    datetime().utc_now()
+  end
+
+  defp datetime() do
+    Application.get_env(:chronoscope, Chronoscope.NTS)[:datetime] || DateTime
+  end
 end
diff --git a/test/chronoscope/nts/client_test.exs b/test/chronoscope/nts/client_test.exs
new file mode 100644
index 0000000..2ab83f3
--- /dev/null
+++ b/test/chronoscope/nts/client_test.exs
@@ -0,0 +1,91 @@
+defmodule Chronoscope.NTS.ClientTest do
+  use Chronoscope.Case
+
+  alias Chronoscope.NTS.Certificate
+  alias Chronoscope.NTS.DateTimeMock
+  alias Chronoscope.NTS.SSLMock
+
+  import Chronoscope.NTS.Client
+  import Mox
+
+  setup :verify_on_exit!
+
+  defp peercert() do
+    :secp256r1
+    |> X509.PrivateKey.new_ec()
+    |> X509.Certificate.self_signed("/C=US/ST=CA/L=San Francisco/O=Acme/CN=Test")
+    |> X509.Certificate.to_der()
+  end
+
+  defp send_ssl_response(response) do
+    send(self(), {:ssl, nil, response})
+    :ok
+  end
+
+  setup _tags do
+    DateTimeMock
+    |> stub(:utc_now, fn -> ~U[2024-03-31 01:23:45Z] end)
+
+    :ok
+  end
+
+  describe "Chronoscope.NTS.Client.init()" do
+    test "initializes successfully" do
+      assert init(%{host: "localhost", port: 3333}) ==
+               {:ok,
+                %{
+                  host: "localhost",
+                  key_establishment_response: {:error, "initializing"},
+                  last_key_establishment: ~U[2024-03-31 01:23:15Z],
+                  port: 3333
+                }}
+    end
+  end
+
+  describe "Chronoscope.NTS.Client.handle_call()" do
+    test ":list" do
+      assert handle_call(:list, nil, %{state: true}) == {:reply, %{state: true}, %{state: true}}
+    end
+
+    test ":key_establishment - cached" do
+      assert handle_call(:key_establishment, nil, %{
+               host: "localhost",
+               key_establishment_response: {:error, "initializing"},
+               last_key_establishment: ~U[2024-03-31 01:23:45Z],
+               port: 3333
+             }) ==
+               {:reply, {:error, "initializing"},
+                %{
+                  host: "localhost",
+                  key_establishment_response: {:error, "initializing"},
+                  last_key_establishment: ~U[2024-03-31 01:23:45Z],
+                  port: 3333
+                }}
+    end
+
+    test ":key_establishment - not cached" do
+      peercert = peercert()
+      peercert_expiration = Certificate.expiration_date(peercert)
+
+      SSLMock
+      |> expect(:connect, fn ~c"localhost", 3333, _, _ -> {:ok, :socket} end)
+      |> expect(:send, fn :socket, _ -> send_ssl_response([]) end)
+      |> expect(:peercert, fn :socket -> {:ok, peercert} end)
+      |> expect(:close, fn :socket -> :ok end)
+
+      assert {:reply, {:ok, %{cert_expiration: ^peercert_expiration}},
+              %{
+                host: "localhost",
+                key_establishment_response: {:ok, %{cert_expiration: ^peercert_expiration}},
+                last_key_establishment: ~U[2024-03-31 01:23:45Z],
+                port: 3333
+              }} =
+               handle_call(:key_establishment, nil, %{
+                 host: "localhost",
+                 key_establishment_response: {:error, "initializing"},
+                 last_key_establishment: ~U[2024-03-31 01:23:00Z],
+                 port: 3333
+               })
+    end
+  end
+end