diff --git a/stl/inc/chrono b/stl/inc/chrono
index 25d3b2ac196..759c2029e1d 100644
--- a/stl/inc/chrono
+++ b/stl/inc/chrono
@@ -1331,15 +1331,16 @@ namespace chrono {
         return *this;
     }
 
-    inline constexpr day _Last_day_table[] = {
-        day{31}, day{28}, day{31}, day{30}, day{31}, day{30}, day{31}, day{31}, day{30}, day{31}, day{30}, day{31}};
+    // To prevent UB by going out of bounds, four extra days with an invalid day are added.
+    inline constexpr day _Last_day_table[] = {day{31}, day{28}, day{31}, day{30}, day{31}, day{30}, day{31}, day{31},
+        day{30}, day{31}, day{30}, day{31}, day{255}, day{255}, day{255}, day{255}};
 
     _NODISCARD constexpr day _Last_day(const year& _Year, const month& _Month) {
         if (_Month == month{2} && _Year.is_leap()) {
             return day{29};
         }
 
-        return _Last_day_table[static_cast<unsigned int>(_Month) - 1];
+        return _Last_day_table[(static_cast<unsigned int>(_Month) - 1) & 0xF];
     }
 
     class year_month_day_last;
diff --git a/tests/std/tests/P0355R7_calendars_and_time_zones_dates/test.cpp b/tests/std/tests/P0355R7_calendars_and_time_zones_dates/test.cpp
index 4bb31c8a863..6a59c863a07 100644
--- a/tests/std/tests/P0355R7_calendars_and_time_zones_dates/test.cpp
+++ b/tests/std/tests/P0355R7_calendars_and_time_zones_dates/test.cpp
@@ -967,6 +967,7 @@ constexpr void year_month_day_last_test() {
     if (is_constant_evaluated()) {
         static_assert((2020y / 1 / last).ok());
         static_assert(!(2020y / 13 / last).ok());
+        static_assert(!(2020y / 0 / last).day().ok()); // implementation-specific assumption, see GH-1647
     } else {
         for (int iy = y_min; iy <= y_max; ++iy) {
             for (auto m = 0u; m <= 255u; ++m) {
@@ -976,6 +977,7 @@ constexpr void year_month_day_last_test() {
                     assert((y / mdl).ok());
                 } else {
                     assert(!(y / mdl).ok());
+                    assert((y / mdl).day().ok() || (y / mdl).day() == day{255}); // implementation-specific assumption
                 }
             }
         }