diff --git a/lib/tz/time_zone_database.ex b/lib/tz/time_zone_database.ex
index 78e46f4..d803d97 100644
--- a/lib/tz/time_zone_database.ex
+++ b/lib/tz/time_zone_database.ex
@@ -136,6 +136,10 @@ defmodule Tz.TimeZoneDatabase do
     div(days * 86_400_000_000 + parts_in_day, 1_000_000)
   end
 
+  defp iso_days_to_gregorian_seconds({days, {parts_in_day, parts_per_day}}) do
+    div(days * parts_per_day + parts_in_day, div(parts_per_day, 86400))
+  end
+
   defp naive_datetime_to_gregorian_seconds(%{calendar: Calendar.ISO, year: year}) when year < 0,
     do: 0
 
diff --git a/test/datetime_math_test.exs b/test/datetime_math_test.exs
new file mode 100644
index 0000000..b307b12
--- /dev/null
+++ b/test/datetime_math_test.exs
@@ -0,0 +1,45 @@
+defmodule DateTimeMathTest do
+  use ExUnit.Case, async: true
+
+  @timezone "America/Fortaleza"
+  @datetime DateTime.from_naive!(~N[2025-04-15 00:00:01.000000], @timezone, Tz.TimeZoneDatabase)
+  @added_second DateTime.add(@datetime, +1, :second, Tz.TimeZoneDatabase)
+  @minus_second DateTime.add(@datetime, -1, :second, Tz.TimeZoneDatabase)
+
+  describe "nanosecond" do
+    test "adds 1 second with nanosecond unit" do
+      amount = System.convert_time_unit(+1, :second, :nanosecond)
+      unit = :nanosecond
+      assert DateTime.add(@datetime, amount, unit, Tz.TimeZoneDatabase) == @added_second
+    end
+
+    test "subtracts 1 second with nanosecond unit" do
+      amount = System.convert_time_unit(-1, :second, :nanosecond)
+      unit = :nanosecond
+      assert DateTime.add(@datetime, amount, unit, Tz.TimeZoneDatabase) == @minus_second
+    end
+  end
+
+  describe "strictly positive integer with units ranging from decisecond (10^-1s) to Planck time (10^-44s)" do
+    # To understand how this units work see:
+    # - https://hexdocs.pm/elixir/1.18.2/System.html#t:time_unit/0
+    # - https://en.wikipedia.org/wiki/Orders_of_magnitude_(time)
+    @units_in_parts_per_second Enum.map(1..44, &System.convert_time_unit(1, :second, 10 ** &1))
+
+    test "adds 1 second" do
+      Enum.each(@units_in_parts_per_second, fn parts_per_second ->
+        amount = +parts_per_second
+        unit = parts_per_second
+        assert DateTime.add(@datetime, amount, unit, Tz.TimeZoneDatabase) == @added_second
+      end)
+    end
+
+    test "subtracts 1 second" do
+      Enum.each(@units_in_parts_per_second, fn parts_per_second ->
+        amount = -parts_per_second
+        unit = parts_per_second
+        assert DateTime.add(@datetime, amount, unit, Tz.TimeZoneDatabase) == @minus_second
+      end)
+    end
+  end
+end
diff --git a/test/dynamic_periods_test.exs b/test/dynamic_periods_test.exs
index 1f4fa91..83a3c64 100644
--- a/test/dynamic_periods_test.exs
+++ b/test/dynamic_periods_test.exs
@@ -1,5 +1,5 @@
 defmodule DynamicPeriodsTest do
-  use ExUnit.Case
+  use ExUnit.Case, async: true
 
   test "dynamic periods" do
     time_zone = "Antarctica/Troll"
diff --git a/test/time_zone_database_test.exs b/test/time_zone_database_test.exs
index a6c1bae..f018d38 100644
--- a/test/time_zone_database_test.exs
+++ b/test/time_zone_database_test.exs
@@ -1,5 +1,5 @@
 defmodule TimeZoneDatabaseTest do
-  use ExUnit.Case
+  use ExUnit.Case, async: true
 
   alias Support.HoloceneCalendar
 
diff --git a/test/updater_test.exs b/test/updater_test.exs
index f12f6e1..6d8eb10 100644
--- a/test/updater_test.exs
+++ b/test/updater_test.exs
@@ -1,6 +1,7 @@
 defmodule UpdaterTest do
-  use ExUnit.Case
+  use ExUnit.Case, async: true
 
+  @tag :updater
   test "updater is only started once" do
     assert {:ok, _} = start_supervised(Tz.UpdatePeriodically)
     assert {:error, _} = start_supervised(Tz.UpdatePeriodically)