diff --git a/libcxx/include/chrono b/libcxx/include/chrono
--- a/libcxx/include/chrono
+++ b/libcxx/include/chrono
@@ -830,6 +830,8 @@
 #include <limits>
 #include <version>
 
+#include <__debug>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
@@ -2454,6 +2456,7 @@
         chrono::day(31), chrono::day(31), chrono::day(30),
         chrono::day(31), chrono::day(30), chrono::day(31)
     };
+    _LIBCPP_ASSERT(ok(), "year_month_day_last::day(): year_month_day_last is invalid");
     return month() != February || !__y.is_leap() ?
         __d[static_cast<unsigned>(month()) - 1] : chrono::day{29};
 }
diff --git a/libcxx/test/libcxx/algorithms/debug_less.pass.cpp b/libcxx/test/libcxx/algorithms/debug_less.pass.cpp
--- a/libcxx/test/libcxx/algorithms/debug_less.pass.cpp
+++ b/libcxx/test/libcxx/algorithms/debug_less.pass.cpp
@@ -14,8 +14,13 @@
 
 // __debug_less checks that a comparator actually provides a strict-weak ordering.
 
+#include <chrono> // Include before defining _LIBCPP_ASSERT: cannot throw in a function marked noexcept.
+
 struct DebugException {};
 
+#ifdef _LIBCPP_ASSERT
+#undef _LIBCPP_ASSERT
+#endif
 #define _LIBCPP_DEBUG 0
 #define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : throw ::DebugException())
 
diff --git a/libcxx/www/cxx2a_status.html b/libcxx/www/cxx2a_status.html
--- a/libcxx/www/cxx2a_status.html
+++ b/libcxx/www/cxx2a_status.html
@@ -439,7 +439,7 @@
 	<tr><td><a href="https://wg21.link/LWG3209">3209</a></td><td>Expression in <tt>year::ok()</tt> returns clause is ill-formed</td><td>Cologne</td><td>Complete</td></tr>
 
 	<tr><td></td><td></td><td></td><td></td></tr>
-	<tr><td><a href="https://wg21.link/LWG3231">3231</a></td><td><tt>year_month_day_last::day</tt> specification does not cover <tt>!ok()</tt> values</td><td>Belfast</td><td></td></tr>
+	<tr><td><a href="https://wg21.link/LWG3231">3231</a></td><td><tt>year_month_day_last::day</tt> specification does not cover <tt>!ok()</tt> values</td><td>Belfast</td><td><i>Nothing to do</i></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3225">3225</a></td><td><tt>zoned_time</tt> converting constructor shall not be <tt>noexcept</tt></td><td>Belfast</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3190">3190</a></td><td><tt>std::allocator::allocate</tt> sometimes returns too little storage</td><td>Belfast</td><td></td></tr>
 	<tr><td><a href="https://wg21.link/LWG3218">3218</a></td><td>Modifier for <tt>%d</tt> parse flag does not match POSIX and <tt>format</tt> specification</td><td>Belfast</td><td></td></tr>