diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa5cf10..b3a0132 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,7 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Grammar additions: `duration` and `relative_date` productions
 - Full pipeline support (lexer, parser, compiler, evaluator, string visitor) with tests
 
-#### Examples:
+#### Examples
 
 ```elixir
 Predicator.evaluate("created_at > 3d ago", %{"created_at" => ~U[2024-01-20 00:00:00Z]})
diff --git a/lib/predicator/duration.ex b/lib/predicator/duration.ex
index e2aaaf7..44a1112 100644
--- a/lib/predicator/duration.ex
+++ b/lib/predicator/duration.ex
@@ -8,10 +8,10 @@ defmodule Predicator.Duration do
   ## Examples
 
       iex> Predicator.Duration.new(days: 3, hours: 8)
-      %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0}
+      %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0, milliseconds: 0}
 
       iex> Predicator.Duration.from_units([{"3", "d"}, {"8", "h"}])
-      {:ok, %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0}}
+      {:ok, %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0, milliseconds: 0}}
 
       iex> Predicator.Duration.to_seconds(%{days: 1, hours: 2, minutes: 30})
       95400
@@ -27,10 +27,10 @@ defmodule Predicator.Duration do
   ## Examples
 
       iex> Predicator.Duration.new(days: 2, hours: 3)
-      %{years: 0, months: 0, weeks: 0, days: 2, hours: 3, minutes: 0, seconds: 0}
+      %{years: 0, months: 0, weeks: 0, days: 2, hours: 3, minutes: 0, seconds: 0, milliseconds: 0}
 
       iex> Predicator.Duration.new()
-      %{years: 0, months: 0, weeks: 0, days: 0, hours: 0, minutes: 0, seconds: 0}
+      %{years: 0, months: 0, weeks: 0, days: 0, hours: 0, minutes: 0, seconds: 0, milliseconds: 0}
   """
   @spec new(keyword()) :: Types.duration()
   def new(opts \\ []) do
@@ -41,7 +41,8 @@ defmodule Predicator.Duration do
       days: Keyword.get(opts, :days, 0),
       hours: Keyword.get(opts, :hours, 0),
       minutes: Keyword.get(opts, :minutes, 0),
-      seconds: Keyword.get(opts, :seconds, 0)
+      seconds: Keyword.get(opts, :seconds, 0),
+      milliseconds: Keyword.get(opts, :milliseconds, 0)
     }
   end
 
@@ -53,7 +54,7 @@ defmodule Predicator.Duration do
   ## Examples
 
       iex> Predicator.Duration.from_units([{"3", "d"}, {"8", "h"}])
-      {:ok, %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0}}
+      {:ok, %{years: 0, months: 0, weeks: 0, days: 3, hours: 8, minutes: 0, seconds: 0, milliseconds: 0}}
 
       iex> Predicator.Duration.from_units([{"invalid", "d"}])
       {:error, "Invalid duration value: invalid"}
@@ -90,7 +91,7 @@ defmodule Predicator.Duration do
 
       iex> duration = Predicator.Duration.new(days: 1)
       iex> Predicator.Duration.add_unit(duration, "h", 3)
-      %{years: 0, months: 0, weeks: 0, days: 1, hours: 3, minutes: 0, seconds: 0}
+      %{years: 0, months: 0, weeks: 0, days: 1, hours: 3, minutes: 0, seconds: 0, milliseconds: 0}
   """
   @spec add_unit(Types.duration(), binary(), non_neg_integer()) :: Types.duration()
   def add_unit(duration, "y", value), do: %{duration | years: duration.years + value}
@@ -101,6 +102,9 @@ defmodule Predicator.Duration do
   def add_unit(duration, "m", value), do: %{duration | minutes: duration.minutes + value}
   def add_unit(duration, "s", value), do: %{duration | seconds: duration.seconds + value}
 
+  def add_unit(duration, "ms", value),
+    do: %{duration | milliseconds: duration.milliseconds + value}
+
   def add_unit(_duration, unit, _value) do
     throw({:error, "Unknown duration unit: #{unit}"})
   end
@@ -131,6 +135,33 @@ defmodule Predicator.Duration do
       Map.get(duration, :years, 0) * 31_536_000
   end
 
+  @doc """
+  Converts a duration to total milliseconds (approximate for months and years).
+
+  Uses approximate conversions:
+  - 1 month = 30 days
+  - 1 year = 365 days
+
+  ## Examples
+
+      iex> Predicator.Duration.to_milliseconds(%{seconds: 1, milliseconds: 500})
+      1500
+
+      iex> Predicator.Duration.to_milliseconds(%{minutes: 1, seconds: 30, milliseconds: 250})
+      90250
+  """
+  @spec to_milliseconds(Types.duration()) :: integer()
+  def to_milliseconds(duration) do
+    Map.get(duration, :milliseconds, 0) +
+      Map.get(duration, :seconds, 0) * 1_000 +
+      Map.get(duration, :minutes, 0) * 60_000 +
+      Map.get(duration, :hours, 0) * 3_600_000 +
+      Map.get(duration, :days, 0) * 86_400_000 +
+      Map.get(duration, :weeks, 0) * 604_800_000 +
+      Map.get(duration, :months, 0) * 2_592_000_000 +
+      Map.get(duration, :years, 0) * 31_536_000_000
+  end
+
   @doc """
   Adds a duration to a Date, returning a Date.
 
@@ -171,6 +202,11 @@ defmodule Predicator.Duration do
       ~U[2024-01-17T14:00:00Z]
   """
   @spec add_to_datetime(DateTime.t(), Types.duration()) :: DateTime.t()
+  def add_to_datetime(datetime, %{milliseconds: ms} = duration) when ms > 0 do
+    total_ms = to_milliseconds(duration)
+    DateTime.add(datetime, total_ms, :millisecond)
+  end
+
   def add_to_datetime(datetime, duration) do
     total_seconds = to_seconds(duration)
     DateTime.add(datetime, total_seconds, :second)
@@ -216,6 +252,11 @@ defmodule Predicator.Duration do
       ~U[2024-01-15T10:30:00Z]
   """
   @spec subtract_from_datetime(DateTime.t(), Types.duration()) :: DateTime.t()
+  def subtract_from_datetime(datetime, %{milliseconds: ms} = duration) when ms > 0 do
+    total_ms = to_milliseconds(duration)
+    DateTime.add(datetime, -total_ms, :millisecond)
+  end
+
   def subtract_from_datetime(datetime, duration) do
     total_seconds = to_seconds(duration)
     DateTime.add(datetime, -total_seconds, :second)
@@ -243,7 +284,8 @@ defmodule Predicator.Duration do
       {:days, "d"},
       {:hours, "h"},
       {:minutes, "m"},
-      {:seconds, "s"}
+      {:seconds, "s"},
+      {:milliseconds, "ms"}
     ]
 
     parts =
diff --git a/lib/predicator/evaluator.ex b/lib/predicator/evaluator.ex
index 9d48f0a..08638ba 100644
--- a/lib/predicator/evaluator.ex
+++ b/lib/predicator/evaluator.ex
@@ -1096,34 +1096,30 @@ defmodule Predicator.Evaluator do
   defp unit_string_to_atom("sec"), do: {:ok, :seconds}
   defp unit_string_to_atom("second"), do: {:ok, :seconds}
   defp unit_string_to_atom("seconds"), do: {:ok, :seconds}
+  defp unit_string_to_atom("ms"), do: {:ok, :milliseconds}
+  defp unit_string_to_atom("millisecond"), do: {:ok, :milliseconds}
+  defp unit_string_to_atom("milliseconds"), do: {:ok, :milliseconds}
   defp unit_string_to_atom(_unknown_unit), do: {:error, :invalid_unit}
 
   @spec add_duration(DateTime.t(), Types.duration()) :: DateTime.t()
+  defp add_duration(datetime, %{milliseconds: ms} = duration) when ms > 0 do
+    total_ms = Predicator.Duration.to_milliseconds(duration)
+    DateTime.add(datetime, total_ms, :millisecond)
+  end
+
   defp add_duration(datetime, duration) do
-    total_seconds = duration_to_seconds(duration)
+    total_seconds = Predicator.Duration.to_seconds(duration)
     DateTime.add(datetime, total_seconds, :second)
   end
 
   @spec subtract_duration(DateTime.t(), Types.duration()) :: DateTime.t()
-  defp subtract_duration(datetime, duration) do
-    total_seconds = duration_to_seconds(duration)
-    DateTime.add(datetime, -total_seconds, :second)
+  defp subtract_duration(datetime, %{milliseconds: ms} = duration) when ms > 0 do
+    total_ms = Predicator.Duration.to_milliseconds(duration)
+    DateTime.add(datetime, -total_ms, :millisecond)
   end
 
-  # Helper function to convert duration to total seconds
-  @spec duration_to_seconds(Types.duration()) :: integer()
-  defp duration_to_seconds(duration) do
-    # Calculate total seconds for all time units (weeks, days, hours, minutes, seconds)
-    # For years and months, we'll approximate using days for now
-    years_in_days = Map.get(duration, :years, 0) * 365
-    months_in_days = Map.get(duration, :months, 0) * 30
-
-    years_in_days * 24 * 3600 +
-      months_in_days * 24 * 3600 +
-      Map.get(duration, :weeks, 0) * 7 * 24 * 3600 +
-      Map.get(duration, :days, 0) * 24 * 3600 +
-      Map.get(duration, :hours, 0) * 3600 +
-      Map.get(duration, :minutes, 0) * 60 +
-      Map.get(duration, :seconds, 0)
+  defp subtract_duration(datetime, duration) do
+    total_seconds = Predicator.Duration.to_seconds(duration)
+    DateTime.add(datetime, -total_seconds, :second)
   end
 end
diff --git a/lib/predicator/lexer.ex b/lib/predicator/lexer.ex
index 4a42065..d5e366c 100644
--- a/lib/predicator/lexer.ex
+++ b/lib/predicator/lexer.ex
@@ -700,20 +700,22 @@ defmodule Predicator.Lexer do
           {:ok, binary(), binary(), binary()} | :no_match
   defp extract_duration_unit(str) do
     cond do
-      # Match unit followed by digits (for sequences like "d8h")
-      match = Regex.run(~r/^(mo)(\d.*)/, str) ->
+      # Match multi-character units first (ms, mo) followed by digits
+      match = Regex.run(~r/^(ms|mo)(\d.*)/, str) ->
         [_full_match, unit, remaining] = match
         {:ok, "", unit, remaining}
 
+      # Match single-character units followed by digits
       match = Regex.run(~r/^([ydhmsw])(\d.*)/, str) ->
         [_full_match, unit, remaining] = match
         {:ok, "", unit, remaining}
 
-      # Match unit at end or followed by non-digits (for cases like "d" or "d ago")
-      match = Regex.run(~r/^(mo)(\D.*|$)/, str) ->
+      # Match multi-character units at end or followed by non-digits (ms, mo)
+      match = Regex.run(~r/^(ms|mo)(\D.*|$)/, str) ->
         [_full_match, unit, remaining] = match
         {:ok, "", unit, remaining}
 
+      # Match single-character units at end or followed by non-digits
       match = Regex.run(~r/^([ydhmsw])(\D.*|$)/, str) ->
         [_full_match, unit, remaining] = match
         {:ok, "", unit, remaining}
@@ -728,6 +730,7 @@ defmodule Predicator.Lexer do
   defp duration_unit?("h"), do: true
   defp duration_unit?("m"), do: true
   defp duration_unit?("s"), do: true
+  defp duration_unit?("ms"), do: true
   defp duration_unit?("w"), do: true
   defp duration_unit?("mo"), do: true
   defp duration_unit?("y"), do: true
diff --git a/lib/predicator/types.ex b/lib/predicator/types.ex
index bb4f205..b2d834b 100644
--- a/lib/predicator/types.ex
+++ b/lib/predicator/types.ex
@@ -31,7 +31,8 @@ defmodule Predicator.Types do
           days: non_neg_integer(),
           hours: non_neg_integer(),
           minutes: non_neg_integer(),
-          seconds: non_neg_integer()
+          seconds: non_neg_integer(),
+          milliseconds: non_neg_integer()
         }
 
   @typedoc """
diff --git a/test/predicator/duration_test.exs b/test/predicator/duration_test.exs
index c7c0d8b..c44c284 100644
--- a/test/predicator/duration_test.exs
+++ b/test/predicator/duration_test.exs
@@ -15,7 +15,8 @@ defmodule Predicator.DurationTest do
                days: 0,
                hours: 0,
                minutes: 0,
-               seconds: 0
+               seconds: 0,
+               milliseconds: 0
              }
     end
 
@@ -29,7 +30,8 @@ defmodule Predicator.DurationTest do
                days: 3,
                hours: 8,
                minutes: 30,
-               seconds: 0
+               seconds: 0,
+               milliseconds: 0
              }
     end
 
@@ -42,7 +44,8 @@ defmodule Predicator.DurationTest do
           days: 4,
           hours: 5,
           minutes: 6,
-          seconds: 7
+          seconds: 7,
+          milliseconds: 123
         )
 
       assert duration.years == 1
@@ -52,6 +55,7 @@ defmodule Predicator.DurationTest do
       assert duration.hours == 5
       assert duration.minutes == 6
       assert duration.seconds == 7
+      assert duration.milliseconds == 123
     end
   end
 
@@ -365,5 +369,124 @@ defmodule Predicator.DurationTest do
       duration = Duration.new(years: 1)
       assert Duration.to_string(duration) == "1y"
     end
+
+    test "formats milliseconds" do
+      duration = Duration.new(milliseconds: 500)
+      assert Duration.to_string(duration) == "500ms"
+    end
+
+    test "formats complex duration with milliseconds" do
+      duration = Duration.new(seconds: 30, milliseconds: 250)
+      assert Duration.to_string(duration) == "30s250ms"
+    end
+  end
+
+  describe "milliseconds support" do
+    test "creates duration with milliseconds only" do
+      duration = Duration.new(milliseconds: 500)
+      assert duration.milliseconds == 500
+    end
+
+    test "adds milliseconds unit" do
+      duration = Duration.new() |> Duration.add_unit("ms", 750)
+      assert duration.milliseconds == 750
+    end
+
+    test "accumulates millisecond values" do
+      duration = Duration.new(milliseconds: 200) |> Duration.add_unit("ms", 300)
+      assert duration.milliseconds == 500
+    end
+
+    test "from_units handles milliseconds" do
+      {:ok, duration} = Duration.from_units([{"500", "ms"}])
+      assert duration.milliseconds == 500
+    end
+
+    test "from_units handles mixed units with milliseconds" do
+      {:ok, duration} = Duration.from_units([{"1", "s"}, {"500", "ms"}])
+      assert duration.seconds == 1
+      assert duration.milliseconds == 500
+    end
+  end
+
+  describe "to_milliseconds/1" do
+    test "converts simple milliseconds" do
+      duration = Duration.new(milliseconds: 500)
+      assert Duration.to_milliseconds(duration) == 500
+    end
+
+    test "converts seconds to milliseconds" do
+      duration = Duration.new(seconds: 2)
+      assert Duration.to_milliseconds(duration) == 2000
+    end
+
+    test "converts mixed seconds and milliseconds" do
+      duration = Duration.new(seconds: 1, milliseconds: 500)
+      assert Duration.to_milliseconds(duration) == 1500
+    end
+
+    test "converts minutes to milliseconds" do
+      duration = Duration.new(minutes: 1, seconds: 30, milliseconds: 250)
+      expected = 1 * 60_000 + 30 * 1_000 + 250
+      assert Duration.to_milliseconds(duration) == expected
+    end
+
+    test "converts hours to milliseconds" do
+      duration = Duration.new(hours: 1)
+      assert Duration.to_milliseconds(duration) == 3_600_000
+    end
+
+    test "converts days to milliseconds" do
+      duration = Duration.new(days: 1)
+      assert Duration.to_milliseconds(duration) == 86_400_000
+    end
+
+    test "converts zero duration" do
+      duration = Duration.new()
+      assert Duration.to_milliseconds(duration) == 0
+    end
+
+    test "converts complex duration to milliseconds" do
+      duration = Duration.new(hours: 1, minutes: 30, seconds: 45, milliseconds: 123)
+      expected = 1 * 3_600_000 + 30 * 60_000 + 45 * 1_000 + 123
+      assert Duration.to_milliseconds(duration) == expected
+    end
+  end
+
+  describe "datetime operations with milliseconds" do
+    test "add_to_datetime uses millisecond precision when milliseconds present" do
+      datetime = ~U[2024-01-15T10:30:00.000Z]
+      duration = Duration.new(seconds: 1, milliseconds: 500)
+      result = Duration.add_to_datetime(datetime, duration)
+      assert result == ~U[2024-01-15T10:30:01.500Z]
+    end
+
+    test "add_to_datetime uses second precision when no milliseconds" do
+      datetime = ~U[2024-01-15T10:30:00.000Z]
+      duration = Duration.new(seconds: 5)
+      result = Duration.add_to_datetime(datetime, duration)
+      assert result == ~U[2024-01-15T10:30:05.000Z]
+    end
+
+    test "subtract_from_datetime uses millisecond precision when milliseconds present" do
+      datetime = ~U[2024-01-15T10:30:02.750Z]
+      duration = Duration.new(seconds: 1, milliseconds: 250)
+      result = Duration.subtract_from_datetime(datetime, duration)
+      assert result == ~U[2024-01-15T10:30:01.500Z]
+    end
+
+    test "subtract_from_datetime uses second precision when no milliseconds" do
+      datetime = ~U[2024-01-15T10:30:05.000Z]
+      duration = Duration.new(seconds: 2)
+      result = Duration.subtract_from_datetime(datetime, duration)
+      assert result == ~U[2024-01-15T10:30:03.000Z]
+    end
+
+    test "millisecond precision with complex durations" do
+      datetime = ~U[2024-01-15T10:30:00.000Z]
+      duration = Duration.new(minutes: 1, seconds: 30, milliseconds: 750)
+      result = Duration.add_to_datetime(datetime, duration)
+      assert result == ~U[2024-01-15T10:31:30.750Z]
+    end
   end
 end