diff --git a/libcxx/test/std/time/time.cal/time.cal.ymwd/time.cal.ymwd.members/op.sys_days.pass.cpp b/libcxx/test/std/time/time.cal/time.cal.ymwd/time.cal.ymwd.members/op.sys_days.pass.cpp
--- a/libcxx/test/std/time/time.cal/time.cal.ymwd/time.cal.ymwd.members/op.sys_days.pass.cpp
+++ b/libcxx/test/std/time/time.cal/time.cal.ymwd/time.cal.ymwd.members/op.sys_days.pass.cpp
@@ -70,5 +70,14 @@
     assert( year_month_weekday{sd} == ymwd); // and back
     }
 
+    {
+    // Index 0 returns 7 weekdays before index 1 and can't be round-tripped.
+    constexpr year_month_weekday ymwd{year{2000}, month{2}, weekday_indexed{std::chrono::Wednesday, 0}};
+    constexpr sys_days sd{ymwd};
+
+    static_assert(sd.time_since_epoch() == days{10957 + 25});
+    static_assert(year_month_weekday{sd} != ymwd); // and back fails
+    }
+
     return 0;
 }