23.1 idiomatic expression in the context of time and date
If the time of the day is indicated without any preposition there is no article, nor there is one if we demand for the time of the day or when we answer. However if the time of the day is used with a preposition, there is an article.
1) Que horas são?
What is the time?
2) São seis e quinze da tarde.
It´s a quarter past six p.m.
3) Quinze paraas cinco da manhã.
It is a quarter to five a.m. .
In case 3) we have a preposition and therefore an article is needed.
For those who speak Spanish. No, it doesn't work as in Spanish. In Spanish there is always an article.
Portuguese: São três e dez.
Spanish: Son las tres y diez.
It doesn't matter where the preposition comes from. Even in the case that the preposition is required by the verb, there is an article.
1) A que hora começam as aulas de Inglês?
At what time does the English lesson starts?
2) As aulas de Inglês começam às quinze para as quatro.
The English lesson starts at a quarter to four.
In 2) there is an article, because começar requires a preposition, a in this case. Remember that a (preposition) and a / as (article) is joined to à / às see 4.1.1.
The article is necessary because horas means time of the date and hours. To put an article or not can make a very big difference.
1) We work until five o' clock.
Trabalhamos até às cinco horas. *
2) We work up to five hours.
Trabalhamos até cinco horas.
It is obvious that the sentences in the table above doesn't mean the same thing. In English there are different words for each context. If this is not the case, another distinguishing mark is needed, for instance an article. May be there is another explanation for the use of the article in this context, but in any case this explanation helps to remember that.
time of the day with preposition
Minha festa de aniversário será às sete em ponto.
My birthday party starts at seven o' clock.
Noite passada fui dormir à meia noite.
Last night I went to bed at midnight.
Tens até às cinco horas da tarde, para tirar as coisas do meu quarto.
You have time until five o' clock in the afternoon to take your stuff out of my room.
Nós trabalhamos até às dez horas.
We work until ten o' clock.
Tinha marcado o despertador para as cinco horas.
He sets the alarm clock to five o' clock.
Entre 3 e 4 de julho há música até depois das cinco horas da manhã.
In the night from the 3rd and 4th of june there will be music up to five o' clock in the morning.