Calendar: Limit the starting day input range to the selected month

Previously we allowed entering any day value greater than one. With this
patch the maximum input value is dynamic based on the selected month and
year.

Userland/Applications/Calendar/AddEventDialog.cpp

@@ -63,8 +63,8 @@ AddEventDialog::AddEventDialog(Core::DateTime date_time, Window* parent_window)
 
     auto& starting_day_combo = middle_container.add<GUI::SpinBox>();
     starting_day_combo.set_fixed_size(40, 20);
+    starting_day_combo.set_range(1, m_date_time.days_in_month());
     starting_day_combo.set_value(m_date_time.day());
-    starting_day_combo.set_min(1);
 
     auto& starting_year_combo = middle_container.add<GUI::SpinBox>();
     starting_year_combo.set_fixed_size(55, 20);
@@ -100,6 +100,16 @@ AddEventDialog::AddEventDialog(Core::DateTime date_time, Window* parent_window)
         done(ExecResult::OK);
     };
 
+    auto update_starting_day_range = [&starting_day_combo, &starting_year_combo, &starting_month_combo]() {
+        auto year = starting_year_combo.value();
+        auto month = starting_month_combo.selected_index();
+
+        starting_day_combo.set_range(1, days_in_month(year, month + 1));
+    };
+
+    starting_year_combo.on_change = [&update_starting_day_range](auto) { update_starting_day_range(); };
+    starting_month_combo.on_change = [&update_starting_day_range](auto, auto) { update_starting_day_range(); };
+
     event_title_textbox.set_focus(true);
 }