diff --git a/include/xo/unit/quantity.hpp b/include/xo/unit/quantity.hpp
index dea30c2b..c19c2fe3 100644
--- a/include/xo/unit/quantity.hpp
+++ b/include/xo/unit/quantity.hpp
@@ -78,28 +78,6 @@ namespace xo {
             // divide_by
             // divide_into
 
-            /* parallel implementation to Quantity<Repr, Int>,
-             * but return type will have dimension computed at compile-time
-             */
-            template <typename Quantity2>
-            static constexpr auto multiply(const quantity & x, const Quantity2 & y) {
-                using r_repr_type = std::common_type_t<typename quantity::repr_type,
-                                                       typename Quantity2::repr_type>;
-                using r_int_type = std::common_type_t<typename quantity::ratio_int_type,
-                                                      typename Quantity2::ratio_int_type>;
-                using r_int2x_type = std::common_type_t<typename quantity::ratio_int2x_type,
-                                                        typename Quantity2::ratio_int2x_type>;
-
-                constexpr auto rr = detail::su_product<r_int_type, r_int2x_type>(x.unit(), y.unit());
-
-                r_repr_type r_scale = (::sqrt(rr.outer_scale_sq_)
-                                       * rr.outer_scale_factor_.template convert_to<r_repr_type>()
-                                       * static_cast<r_repr_type>(x.scale())
-                                       * static_cast<r_repr_type>(y.scale()));
-
-                return quantity<r_repr_type, r_int_type, rr.natural_unit_, r_int2x_type>(r_scale);
-            }
-
             // divide
             // add
             // subtract
@@ -127,6 +105,53 @@ namespace xo {
             Repr scale_ = Repr{};
         };
 
+        struct quantity_util {
+            /* parallel implementation to Quantity<Repr, Int>,
+             * but return type will have dimension computed at compile-time
+             */
+            template <typename Q1, typename Q2>
+            requires (quantity_concept<Q1>
+                      && quantity_concept<Q2>
+                      && Q1::always_constexpr_unit
+                      && Q2::always_constexpr_unit)
+            static constexpr auto multiply(Q1 x, Q2 y) {
+                using r_repr_type = std::common_type_t<typename Q1::repr_type,
+                                                       typename Q2::repr_type>;
+                using r_int_type = std::common_type_t<typename Q1::ratio_int_type,
+                                                      typename Q2::ratio_int_type>;
+                using r_int2x_type = std::common_type_t<typename Q1::ratio_int2x_type,
+                                                        typename Q2::ratio_int2x_type>;
+
+                constexpr auto rr = detail::su_product<r_int_type, r_int2x_type>(x.unit(), y.unit());
+
+                r_repr_type r_scale = (((rr.outer_scale_sq_ == 1.0)
+                                        ? 1.0
+                                        : ::sqrt(rr.outer_scale_sq_))
+                                       * rr.outer_scale_factor_.template convert_to<r_repr_type>()
+                                       * static_cast<r_repr_type>(x.scale())
+                                       * static_cast<r_repr_type>(y.scale()));
+
+                return quantity<r_repr_type,
+                                r_int_type,
+                                rr.natural_unit_,
+                                r_int2x_type
+                                >(r_scale);
+            }
+        };
+
+        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
+         **/
+        template <typename Q1, typename Q2>
+        requires (quantity_concept<Q1>
+                  && quantity_concept<Q2>
+                  && Q1::always_constexpr_unit
+                  && Q2::always_constexpr_unit)
+        constexpr auto
+        operator* (const Q1 & x, const Q2 & y)
+        {
+            return quantity_util::multiply(x, y);
+        }
+
         namespace qty {
             // ----- mass -----
 
@@ -179,6 +204,8 @@ namespace xo {
             inline constexpr auto volatility_250d(double x) { return quantity<double, std::int64_t, nu::volatility_250d>(x); }
             inline constexpr auto volatility_360d(double x) { return quantity<double, std::int64_t, nu::volatility_360d>(x); }
         }
+
+        /* reminder: see [quantity_ops.hpp] for operator* etc */
     } /*namespace qty*/
 } /*namespace xo*/
 
diff --git a/include/xo/unit/quantity_ops.hpp b/include/xo/unit/quantity_ops.hpp
index a8c806b3..8fa532c3 100644
--- a/include/xo/unit/quantity_ops.hpp
+++ b/include/xo/unit/quantity_ops.hpp
@@ -6,20 +6,9 @@
 #pragma once
 
 #include "quantity_concept.hpp"
-//#include <cstdint>
 
 namespace xo {
     namespace qty {
-        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
-         **/
-        template <typename Quantity, typename Quantity2>
-        requires quantity_concept<Quantity> && quantity_concept<Quantity2>
-        constexpr auto
-        operator* (const Quantity & x, const Quantity2 & y)
-        {
-            return Quantity::multiply(x, y);
-        }
-
         /** note: does not require unit scaling,  so constexpr with c++23 **/
         template <typename Dimensionless, typename Quantity>
         requires std::is_arithmetic_v<Dimensionless> && quantity_concept<Quantity>
diff --git a/include/xo/unit/scaled_unit.hpp b/include/xo/unit/scaled_unit.hpp
index f9cc8245..dce63a90 100644
--- a/include/xo/unit/scaled_unit.hpp
+++ b/include/xo/unit/scaled_unit.hpp
@@ -50,6 +50,8 @@ namespace xo {
 
             constexpr auto nanogram = detail::make_unit_rescale_result<std::int64_t>(nu::nanogram);
             constexpr auto microgram = detail::make_unit_rescale_result<std::int64_t>(nu::microgram);
+
+            constexpr auto millisecond = detail::make_unit_rescale_result<std::int64_t>(nu::millisecond);
         }
 
         namespace detail {
diff --git a/include/xo/unit/xquantity.hpp b/include/xo/unit/xquantity.hpp
index f2d49b2d..cf22e13c 100644
--- a/include/xo/unit/xquantity.hpp
+++ b/include/xo/unit/xquantity.hpp
@@ -230,7 +230,8 @@ namespace xo {
         template <typename Repr = double,
                   typename Int = std::int64_t>
         inline constexpr xquantity<Repr, Int>
-        unit_qty(const scaled_unit<Int> & u) {
+        unit_qty(const scaled_unit<Int> & u)
+        {
             return xquantity<Repr, Int>
                 (u.outer_scale_factor_.template convert_to<double>() * ::sqrt(u.outer_scale_sq_),
                  u.natural_unit_);
@@ -245,6 +246,18 @@ namespace xo {
             return xquantity<Repr, Int>(1.0, nu);
         }
 
+        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
+         **/
+        template <typename Q1, typename Q2>
+        requires (quantity_concept<Q1>
+                  && quantity_concept<Q2>
+                  && (!Q1::always_constexpr_unit || !Q2::always_constexpr_unit))
+        constexpr auto
+        operator* (const Q1 & x, const Q2 & y)
+        {
+            return Q1::multiply(x, y);
+        }
+
         /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
          **/
         template <typename Quantity, typename Quantity2>
diff --git a/utest/quantity.test.cpp b/utest/quantity.test.cpp
index b63fa1b5..ea0e9712 100644
--- a/utest/quantity.test.cpp
+++ b/utest/quantity.test.cpp
@@ -2,7 +2,7 @@
 
 #include "xo/unit/quantity.hpp"
 #include "xo/unit/quantity_iostream.hpp"
-#include "xo/unit/quantity2_concept.hpp"
+#include "xo/unit/quantity_concept.hpp"
 #include "xo/indentlog/scope.hpp"
 #include <catch2/catch.hpp>