David , it is worth noting that when we say "добрый день", "доброе утро", if we can say so, we only state the fact. And if we say "добр-ого утра!", "добр-ого дня!" and "добр-ого вечера!", then this is already a wish-of-goodness form (for the morning, for the day, for the evening).
The last form (the wish-of-goodness form) and the endings sound the same in all three cases. What is more convenient for you, learning the language - in all cases the same ending.