diff --git a/CHANGELOG.md b/CHANGELOG.md
index fbeada7..9f618ae 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,7 +20,7 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
   XSD spec 
 - the logical operators and the Effective Boolean Value (EBV) coercion algorithm 
   from the XPath and SPARQL specs on `RDF.Boolean`
-- `RDF.DateTime.now/0`
+- various functions on the `RDF.DateTime` and `RDF.Time` datatypes
 - `RDF.Term.equal?/2` and `RDF.Term.equal_value?/2`
 - `RDF.LangString.match_language?/2`
 - possibility to configure an application-specific default base IRI; for now it 
diff --git a/lib/rdf/datatypes/date_time.ex b/lib/rdf/datatypes/date_time.ex
index 431a7de..019cd0b 100644
--- a/lib/rdf/datatypes/date_time.ex
+++ b/lib/rdf/datatypes/date_time.ex
@@ -5,6 +5,8 @@ defmodule RDF.DateTime do
 
   use RDF.Datatype, id: RDF.Datatype.NS.XSD.dateTime
 
+  import RDF.Literal.Guards
+
 
   def now() do
     new(DateTime.utc_now())
@@ -59,20 +61,35 @@ defmodule RDF.DateTime do
   end
 
   def tz(%Literal{value: %NaiveDateTime{}}), do: ""
-  
+
   def tz(datetime_literal) do
     if valid?(datetime_literal) do
-      lexical = lexical(datetime_literal)
-      case Regex.run(~r/([+-])(\d\d:\d\d)/, lexical) do
-        [_, sign, zone] ->
-          sign <> zone
-        _ ->
-          if String.ends_with?(lexical, "Z") do
-            "Z"
-          else
-            ""
-          end
-      end
+      datetime_literal
+      |> lexical()
+      |> RDF.DateTimeUtils.tz()
+    end
+  end
+
+  @doc """
+  Converts a datetime literal to a canonical string, preserving the zone information.
+  """
+  def canonical_lexical_with_zone(%Literal{datatype: datatype} = literal)
+      when is_xsd_datetime(datatype) do
+    case tz(literal) do
+      nil ->
+        nil
+
+      zone when zone in ["Z", "", "+00:00"] ->
+        canonical_lexical(literal.value)
+
+      zone ->
+        literal
+        |> lexical()
+        |> String.replace_trailing(zone, "Z")
+        |> DateTime.from_iso8601()
+        |> elem(1)
+        |> canonical_lexical()
+        |> String.replace_trailing("Z", zone)
     end
   end
 
diff --git a/lib/rdf/datatypes/date_time_utils.ex b/lib/rdf/datatypes/date_time_utils.ex
new file mode 100644
index 0000000..67925d2
--- /dev/null
+++ b/lib/rdf/datatypes/date_time_utils.ex
@@ -0,0 +1,17 @@
+defmodule RDF.DateTimeUtils do
+  @moduledoc false
+
+  def tz(string) do
+    case Regex.run(~r/([+-])(\d\d:\d\d)/, string) do
+      [_, sign, zone] ->
+        sign <> zone
+      _ ->
+        if String.ends_with?(string, "Z") do
+          "Z"
+        else
+          ""
+        end
+    end
+  end
+
+end
diff --git a/lib/rdf/datatypes/time.ex b/lib/rdf/datatypes/time.ex
index e7f6aae..48502ae 100644
--- a/lib/rdf/datatypes/time.ex
+++ b/lib/rdf/datatypes/time.ex
@@ -5,6 +5,8 @@ defmodule RDF.Time do
 
   use RDF.Datatype, id: RDF.Datatype.NS.XSD.time
 
+  import RDF.Literal.Guards
+
   @grammar ~r/\A(\d{2}:\d{2}:\d{2}(?:\.\d+)?)((?:[\+\-]\d{2}:\d{2})|UTC|GMT|Z)?\Z/
   @tz_grammar ~r/\A(?:([\+\-])(\d{2}):(\d{2}))\Z/
 
@@ -76,4 +78,35 @@ defmodule RDF.Time do
     canonical_lexical(value) <> "Z"
   end
 
+  def tz(time_literal) do
+    if valid?(time_literal) do
+      time_literal
+      |> lexical()
+      |> RDF.DateTimeUtils.tz()
+    end
+  end
+
+
+  @doc """
+  Converts a time literal to a canonical string, preserving the zone information.
+  """
+  def canonical_lexical_with_zone(%Literal{datatype: datatype} = literal)
+      when is_xsd_time(datatype) do
+    case tz(literal) do
+      nil ->
+        nil
+
+      zone when zone in ["Z", "", "+00:00"] ->
+        canonical_lexical(literal.value)
+
+      zone ->
+        literal
+        |> lexical()
+        |> String.replace_trailing(zone, "")
+        |> Time.from_iso8601!()
+        |> canonical_lexical()
+        |> Kernel.<>(zone)
+    end
+  end
+
 end
diff --git a/test/unit/datatypes/date_time_test.exs b/test/unit/datatypes/date_time_test.exs
index 425caae..14f92a0 100644
--- a/test/unit/datatypes/date_time_test.exs
+++ b/test/unit/datatypes/date_time_test.exs
@@ -123,4 +123,12 @@ defmodule RDF.DateTimeTest do
     end
   end
 
+  test "canonical_lexical_with_zone/1" do
+    assert RDF.date_time(~N[2010-01-01T12:34:56])     |> DateTime.canonical_lexical_with_zone() == "2010-01-01T12:34:56"
+    assert RDF.date_time("2010-01-01T12:34:56")       |> DateTime.canonical_lexical_with_zone() == "2010-01-01T12:34:56"
+    assert RDF.date_time("2010-01-01T00:00:00+00:00") |> DateTime.canonical_lexical_with_zone() == "2010-01-01T00:00:00Z"
+    assert RDF.date_time("2010-01-01T01:00:00+01:00") |> DateTime.canonical_lexical_with_zone() == "2010-01-01T01:00:00+01:00"
+    assert RDF.date_time("2010-01-01 01:00:00+01:00") |> DateTime.canonical_lexical_with_zone() == "2010-01-01T01:00:00+01:00"
+  end
+
 end