Ensure moon phase doesn't change at night

This was already theoretically implemented, but it didn't work.  Fix it
and add a test.

src/calendar.cpp

@@ -63,8 +63,9 @@ moon_phase get_moon_phase( const time_point &p )
     const time_duration moon_phase_duration =
         calendar::season_from_default_ratio() * synodic_month;
     // Switch moon phase at noon so it stays the same all night
-    const time_duration current_day = ( p - calendar::turn_zero ) + 1_days / 2;
-    const double phase_change = current_day / moon_phase_duration;
+    const int num_middays = to_days<int>( p - calendar::turn_zero + 1_days / 2 );
+    const time_duration nearest_midnight = num_middays * 1_days;
+    const double phase_change = nearest_midnight / moon_phase_duration;
     const int current_phase = static_cast<int>( round( phase_change * MOON_PHASE_MAX ) ) %
                               static_cast<int>( MOON_PHASE_MAX );
     return static_cast<moon_phase>( current_phase );

tests/calendar_test.cpp

@@ -22,3 +22,17 @@ TEST_CASE( "moon_phases_take_28_days", "[calendar]" )
     CHECK( ( get_moon_phase( first_time ) == get_moon_phase( later_29_days ) ||
              get_moon_phase( first_time ) == get_moon_phase( later_30_days ) ) );
 }
+
+TEST_CASE( "moon_phase_changes_at_noon", "[calendar]" )
+{
+    // This test only makes sense if the seasons are set to the default length
+    REQUIRE( calendar::season_from_default_ratio() == Approx( 1.0f ) );
+
+    const int num_days = GENERATE( take( 100, random( 0, 1000 ) ) );
+    const time_point midnight = calendar::turn_zero + time_duration::from_days( num_days );
+    const time_point earlier_11_hours = midnight - 11_hours;
+    const time_point later_11_hours = midnight + 11_hours;
+
+    CAPTURE( num_days );
+    CHECK( get_moon_phase( earlier_11_hours ) == get_moon_phase( later_11_hours ) );
+}