Time a las or son las
Is a las diez a la noche correct or is it son las diez a la noche?
4 Answers
It depends on the context as well.
¿Que hora es? Son las ocho.
¿A que hora comienza la pelicula? Comienza a las ocho.
As for morning, afternoon, or evening/night, I've learned that it can be "de la" or "en la." My audio course does not do a good job of explaining any rules of when to use "de" or "en."
What time is it? - It is twelve thirty.
¿Qué hora es? - Son las doce y media.
¿Qué hora son? (can vary by country) - Son las doce y media.
de la mañana - (in the morning)
de la tarde - (in the afternoon), or
de la noche - (for in the evening)
¿Qué hora es? (11:45pm) Es cuarto para las doce de la noche
I'm just going to add a little bit to what has been said.
The expression "a la una" or "a las dos, tres, etc." means "at" two or three o'clock.
"A" in this context means "at".
Now the expressions " de la mañana, de la tarde, and de la noche" are the equivalent of the English "am, pm" etc. "De la mañana" is the only "a.m." "De la tarde" and "de la noche" are the "pm's" De la noche usually begins at sunset, but varies regionally.
Por la mañana, por la tarde, por la noche, have nothing to to with clock time. These are general expressions for "in the morning, in the after noon, and in the evening or at night."
"En la mañana, en la tarde, etc." are beginning to be heard, but are regarded by the textbooks as being incorrect.
Remember this is not the whole answer. I am merely adding to what has already been written by the other members.

Time is always given using the verb Ser
¿Que hora es? - What hour is it ?
Son las cinco y veinticinco - They (the hours) are five and twentyfive (5.25).
Son las nueve menos cuarto (8.45).They are the nine minus a quarter (8.45).
Times surrounding one o clock are slightly different in that the hour of one is singular.
Es la una. = It is the one = Its 1 o clock Es la una y diez = It is the one and ten = Its 10 past 1. Es la una menos cuarto = Its the 1 minus (a) quarter = Quarter to one.
Add on de la / manana / tarde / noche to indicate the appropriate part of the day if necessary.
Es la una de la manana (así que ¡vuelve a la cama! )