# UNDF: UNDF-2026-000001090
# UNDF:
--- a/src/calendar/gui/e-date-time-list.c
+++ b/src/calendar/gui/e-date-time-list.c
@@ -56,6 +56,7 @@ struct _EDateTimeListPrivate {
 	GList *list;
 	gboolean use_24_hour_format;
 	gint stamp;
+	GHashTable *date_set; /* ISO-date string keys for O(1) dedup */
 };

 /* EDateTimeList is a GtkTreeModel implementation that stores ICalTime
@@ -580,8 +580,21 @@ e_date_time_list_append (EDateTimeList *date_time_list,
 	g_return_if_fail (i_cal_time_is_valid_time ((ICalTime *) itt));

-	if (g_list_find_custom (date_time_list->priv->list, itt, (GCompareFunc) compare_datetime) == NULL) {
+	if (date_time_list->priv->date_set == NULL)
+		date_time_list->priv->date_set = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL);
+
+	/* Build a canonical key: YYYYMMDD or YYYYMMDDTHHMMSS[Z] */
+	gchar *key = i_cal_time_as_ical_string ((ICalTime *) itt);
+	if (key == NULL)
+		return;
+
+	if (!g_hash_table_contains (date_time_list->priv->date_set, key)) {
+		g_hash_table_add (date_time_list->priv->date_set, key); /* takes ownership */
 		date_time_list->priv->list = g_list_append (date_time_list->priv->list, i_cal_time_clone (itt));
 		row_added (date_time_list, g_list_length (date_time_list->priv->list) - 1);
+	} else {
+		g_free (key);
 	}

 	if (iter) {
@@ -598,6 +611,10 @@ e_date_time_list_clear (EDateTimeList *date_time_list)
 	g_return_if_fail (E_IS_DATE_TIME_LIST (date_time_list));

+	if (date_time_list->priv->date_set) {
+		g_hash_table_destroy (date_time_list->priv->date_set);
+		date_time_list->priv->date_set = NULL;
+	}
+
 	g_list_free_full (date_time_list->priv->list, g_object_unref);
 	date_time_list->priv->list = NULL;
 	date_time_list->priv->stamp++;