diff --git a/lib/trinity/sim_system.ex b/lib/trinity/sim_system.ex
index dfa6603..677e48b 100644
--- a/lib/trinity/sim_system.ex
+++ b/lib/trinity/sim_system.ex
@@ -6,15 +6,22 @@ defmodule Trinity.SimSystem do
   defp get_sim, do: Process.get(simulation_key())
 
   @spec monotonic_time :: integer
-  def monotonic_time do
+  def monotonic_time(unit \\ :native) do
     case get_sim() do
-      nil -> System.monotonic_time()
-      _sim -> sim_monotonic_time()
+      nil -> System.monotonic_time(unit)
+      _sim -> sim_monotonic_time(unit)
     end
   end
 
-  defp sim_monotonic_time do
+  defp sim_monotonic_time(unit) do
     %{now: now} = get_sim()
-    :atomics.get(now, 1)
+
+    time = :atomics.get(now, 1)
+    case unit do
+      :native -> time
+      :millisecond -> time
+      :microsecond -> time * 1000
+      _ -> raise ArgumentError, "Unsupported unit #{inspect(unit)}"
+    end
   end
 end