A Los Nueve

[Bobbert]

A Mexican movie on YouTube from the 1960’s about a rich aunt from Spain who owns a wealthy estate in Mexico and is meeting her “supposed nephew” from Spain for the first time, called “El Falso Heredero” (3/6) at 12:14 and 14:08:

In one scene, the rich aunt, upon meeting her nephew for the first time, says to her nephew:
“La cena es a los nueve.”

In another scene, when the nephew is not at the dining table at nine o’clock, the rich aunt asks the butler:
“¿Avisaste a mi sobrino que la cena es a los nueve?”

What does “a los nueve” refer to. Why did she refer to the specified time as “a los nueve”? I would have thought to say “a las nueve.”

I don't think I heard the phrase wrong, so the YouTube chapter and time stamps are provided above so you can hear them.

Any explanation is appreciated.