UTC times are unique: when a positive leap second is inserted, 23:59:60 is added; when a negative leap second is inserted, 23:59:59 is skipped. Unix timestamps

UTC times are unique: when a positive leap second is inserted, 23:59:60 is added; when a negative leap second is inserted, 23:59:59 is skipped. Unix timestamps, on the other hand, are not unique.