From fd485c5383dd8480a04feaaa31161ae30bc3d3dd Mon Sep 17 00:00:00 2001
From: Howard Hinnant <howard.hinnant@gmail.com>
Date: Tue, 4 Aug 2015 18:48:48 -0400
Subject: [PATCH] [X] Make months and years not convertible to seconds: * The
 Gregorian average of months and years is an   integral number of seconds. *
 This patch slightly tweaks the average to make it   not an integral number of
 seconds, and thus these   units no longer implicitly convert to seconds. *
 This will flag naive  mixing of years, months and seconds   as a compile time
 error.

---
 date.h                            |  3 +-
 test/date_test/durations.pass.cpp | 48 +++++++++++++++++++++++++++++--
 2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/date.h b/date.h
index f7bd09e..49041d1 100644
--- a/date.h
+++ b/date.h
@@ -45,7 +45,8 @@ using weeks = std::chrono::duration
     <int, std::ratio_multiply<std::ratio<7>, days::period>>;
 
 using years = std::chrono::duration
-    <int, std::ratio_multiply<std::ratio<146097, 400>, days::period>>;
+    <int, std::ratio_subtract<std::ratio_multiply<std::ratio<146097, 400>, days::period>,
+                              std::ratio<1, 7>>>;
 
 using months = std::chrono::duration
     <int, std::ratio_divide<years::period, std::ratio<12>>>;
diff --git a/test/date_test/durations.pass.cpp b/test/date_test/durations.pass.cpp
index 9c01009..9be8f91 100644
--- a/test/date_test/durations.pass.cpp
+++ b/test/date_test/durations.pass.cpp
@@ -25,9 +25,53 @@
 #include <type_traits>
 
 static_assert(date::days{1} == std::chrono::seconds{86400}, "");
+static_assert(date::days{1} == std::chrono::hours{24}, "");
+static_assert( std::is_convertible<date::days, std::chrono::seconds>{}, "");
+static_assert( std::is_convertible<date::days, std::chrono::minutes>{}, "");
+static_assert( std::is_convertible<date::days, std::chrono::hours>{}, "");
+static_assert(!std::is_convertible<date::days, date::weeks>{}, "");
+static_assert(!std::is_convertible<date::days, date::months>{}, "");
+static_assert(!std::is_convertible<date::days, date::years>{}, "");
+
 static_assert(date::weeks{1} == std::chrono::seconds{604800}, "");
-static_assert(date::months{1} == std::chrono::seconds{2629746}, "");
-static_assert(date::years{1} == std::chrono::seconds{31556952}, "");
+static_assert(date::weeks{1} == date::days{7}, "");
+static_assert( std::is_convertible<date::weeks, std::chrono::seconds>{}, "");
+static_assert( std::is_convertible<date::weeks, std::chrono::minutes>{}, "");
+static_assert( std::is_convertible<date::weeks, std::chrono::hours>{}, "");
+static_assert( std::is_convertible<date::weeks, date::days>{}, "");
+static_assert(!std::is_convertible<date::weeks, date::months>{}, "");
+static_assert(!std::is_convertible<date::weeks, date::years>{}, "");
+
+static_assert(
+    std::chrono::seconds{2629745} < date::months{1} &&
+                                    date::months{1} < std::chrono::seconds{2629746}, "");
+static_assert(date::days{30} < date::months{1} && date::months{1} < date::days{31}, "");
+static_assert(date::weeks{4} < date::months{1} && date::months{1} < date::weeks{5}, "");
+static_assert(!std::is_convertible<date::months, date::years>{}, "");
+static_assert(!std::is_convertible<date::months, date::weeks>{}, "");
+static_assert(!std::is_convertible<date::months, date::days>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::hours>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::minutes>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::seconds>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::milliseconds>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::microseconds>{}, "");
+static_assert(!std::is_convertible<date::months, std::chrono::nanoseconds>{}, "");
+
+static_assert(
+    std::chrono::seconds{31556951} < date::years{1} &&
+                                     date::years{1} < std::chrono::seconds{31556952}, "");
+static_assert(date::days{365} < date::years{1} && date::years{1} < date::days{366}, "");
+static_assert(date::weeks{52} < date::years{1} && date::years{1} < date::weeks{53}, "");
+static_assert(date::years{1} == date::months{12}, "");
+static_assert( std::is_convertible<date::years, date::months>{}, "");
+static_assert(!std::is_convertible<date::years, date::weeks>{}, "");
+static_assert(!std::is_convertible<date::years, date::days>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::hours>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::minutes>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::seconds>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::milliseconds>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::microseconds>{}, "");
+static_assert(!std::is_convertible<date::years, std::chrono::nanoseconds>{}, "");
 
 static_assert(std::is_same<date::day_point::duration, date::days>{}, "");