diff --git a/test/microwaveprop/weather/grid_cache_test.exs b/test/microwaveprop/weather/grid_cache_test.exs
index 937eb9b8..0c82ad62 100644
--- a/test/microwaveprop/weather/grid_cache_test.exs
+++ b/test/microwaveprop/weather/grid_cache_test.exs
@@ -221,4 +221,18 @@ defmodule Microwaveprop.Weather.GridCacheTest do
       assert {:ok, _} = GridCache.fetch(~U[2026-04-12 11:00:00Z])
     end
   end
+
+  describe "prune_older_than/1" do
+    test "removes expired valid_times" do
+      GridCache.put(~U[2026-04-12 10:00:00Z], [%{lat: 32.0, lon: -97.0}])
+      GridCache.put(~U[2026-04-12 11:00:00Z], [%{lat: 32.0, lon: -97.0}])
+
+      past = ~U[2026-04-12 10:30:00Z]
+      removed = GridCache.prune_older_than(past)
+      assert removed == 1
+
+      assert GridCache.fetch(~U[2026-04-12 10:00:00Z]) == :miss
+      assert {:ok, _} = GridCache.fetch(~U[2026-04-12 11:00:00Z])
+    end
+  end
 end