diff --git a/test/duration_expr_test.exs b/test/duration_expr_test.exs
new file mode 100644
index 00000000..85ba43ea
--- /dev/null
+++ b/test/duration_expr_test.exs
@@ -0,0 +1,89 @@
+# SPDX-FileCopyrightText: 2019 ash_postgres contributors <https://github.com/ash-project/ash_postgres/graphs/contributors>
+#
+# SPDX-License-Identifier: MIT
+
+defmodule AshPostgres.DurationExprTest do
+  use AshPostgres.RepoCase, async: false
+  alias AshPostgres.Test.Post
+
+  require Ash.Query
+
+  defp post!(title, created_at) do
+    Post
+    |> Ash.Changeset.for_create(:create, %{title: title, created_at: created_at})
+    |> Ash.create!()
+  end
+
+  describe "ago/1" do
+    test "filters on a duration" do
+      post!("old", DateTime.add(DateTime.utc_now(), -60, :day))
+      post!("recent", DateTime.utc_now())
+
+      assert [%Post{title: "recent"}] =
+               Post
+               |> Ash.Query.filter(created_at > ago(^Duration.new!(day: 30)))
+               |> Ash.read!()
+    end
+
+    test "filters on a multi-unit duration" do
+      post!("old", DateTime.add(DateTime.utc_now(), -60, :day))
+      post!("recent", DateTime.add(DateTime.utc_now(), -20, :day))
+
+      assert [%Post{title: "recent"}] =
+               Post
+               |> Ash.Query.filter(created_at > ago(^Duration.new!(month: 1, day: 2)))
+               |> Ash.read!()
+    end
+  end
+
+  describe "from_now/1" do
+    test "filters on a duration" do
+      post!("soon", DateTime.add(DateTime.utc_now(), 10, :day))
+      post!("later", DateTime.add(DateTime.utc_now(), 60, :day))
+
+      assert [%Post{title: "soon"}] =
+               Post
+               |> Ash.Query.filter(created_at < from_now(^Duration.new!(day: 30)))
+               |> Ash.read!()
+    end
+  end
+
+  describe "datetime_add/2" do
+    test "adds a duration to a datetime" do
+      now = DateTime.utc_now()
+      post!("before", DateTime.add(now, 3, :day))
+      post!("after", DateTime.add(now, 30, :day))
+
+      assert [%Post{title: "before"}] =
+               Post
+               |> Ash.Query.filter(created_at < datetime_add(^now, ^Duration.new!(day: 7)))
+               |> Ash.read!()
+    end
+
+    test "a negative duration subtracts" do
+      now = DateTime.utc_now()
+      post!("old", DateTime.add(now, -30, :day))
+      post!("recent", now)
+
+      assert [%Post{title: "recent"}] =
+               Post
+               |> Ash.Query.filter(created_at > datetime_add(^now, ^Duration.new!(day: -7)))
+               |> Ash.read!()
+    end
+  end
+
+  describe "date_add/2" do
+    test "adds a duration to a date, and stays a date" do
+      today = Date.utc_today()
+      post!("old", DateTime.add(DateTime.utc_now(), -60, :day))
+      post!("recent", DateTime.utc_now())
+
+      assert [%Post{title: "recent"}] =
+               Post
+               |> Ash.Query.filter(
+                 type(created_at, :date) >= date_add(^today, ^Duration.new!(day: -30))
+               )
+               |> Ash.read!()
+    end
+  end
+end