Voted Best Answer
Mar 21, 2011 - 09:30 AM
It's subtle, but there is a slight difference between the two sentences you posted as examples. In the first one, the time is being specified ("a la una"). In the second example, it's not. Whenever you specify an hour and then the less specific time of day (mañana, tarde, noche), you always use "de" to connect the two (una de la mañana, nueve de la noche). So the "en" isn't really "becoming de." It's just that you're forming a different structure.