diff --git a/include/etl/private/chrono/day.h b/include/etl/private/chrono/day.h
index 60bdfe44..6dfb15f6 100644
--- a/include/etl/private/chrono/day.h
+++ b/include/etl/private/chrono/day.h
@@ -80,7 +80,8 @@ namespace etl
       }
 
       //***********************************************************************
-      /// Pre-increment operator
+      /// Assignment operator
+      //***********************************************************************
       template <typename TToDuration, typename TValue2, typename TPeriod2>
       ETL_CONSTEXPR14 etl::chrono::day& operator =(const etl::chrono::duration<TValue2, TPeriod2>& rhs)
       {
@@ -89,6 +90,8 @@ namespace etl
         return *this;
       }
 
+      //***********************************************************************
+      /// Pre-increment operator
       //***********************************************************************
       ETL_CONSTEXPR14 etl::chrono::day& operator ++() ETL_NOEXCEPT
       {
diff --git a/include/etl/private/chrono/duration.h b/include/etl/private/chrono/duration.h
index 9aa84807..b16ed6ba 100644
--- a/include/etl/private/chrono/duration.h
+++ b/include/etl/private/chrono/duration.h
@@ -366,12 +366,6 @@ namespace etl
         return TToDuration(static_cast<to_rep>((ct_count * ct_num) / ct_den));
       }
     }
-
-    template <typename T>
-    struct is_duration : etl::false_type {};
-
-    template <typename TRep, typename TPeriod>
-    struct is_duration<etl::chrono::duration<TRep, TPeriod>> : etl::true_type {};
   }
 
   //*************************************************************************
diff --git a/include/etl/private/chrono/hh_mm_ss.h b/include/etl/private/chrono/hh_mm_ss.h
index 3dac6a5f..d044c726 100644
--- a/include/etl/private/chrono/hh_mm_ss.h
+++ b/include/etl/private/chrono/hh_mm_ss.h
@@ -44,7 +44,8 @@ namespace etl
   template <typename TDuration>
   ETL_NODISCARD
   ETL_CONSTEXPR14 
-  typename etl::enable_if<etl::chrono::is_duration<TDuration>::value, TDuration>::type
+  //typename etl::enable_if<etl::chrono::is_duration<TDuration>::value, TDuration>::type
+  typename etl::enable_if<etl::is_specialization<TDuration, etl::chrono::duration>::value, TDuration>::type
     absolute(TDuration dur) ETL_NOEXCEPT
   {
     return TDuration((dur.count() < 0) ? -dur.count() : dur.count());
@@ -86,13 +87,13 @@ namespace etl
       struct calculate_fractional_width
       {
         static constexpr int value = (TDur::period::den == 1)
-          ? 0
-          : fractional_width_helper<TDur::period::den>::value;
+                                     ? 0
+                                     : fractional_width_helper<TDur::period::den>::value;
       };
 
     public:
 
-      ETL_STATIC_ASSERT(etl::chrono::is_duration<TDuration>::value, "TDuration is not a etl::chrono::duration type");
+      ETL_STATIC_ASSERT((etl::is_specialization<TDuration, etl::chrono::duration>::value), "TDuration is not a etl::chrono::duration type");
 
       static constexpr int fractional_width = calculate_fractional_width<TDuration>::value;