The number An or a it is only apocoped (suppresses a sound) if it precedes it masculine nouns (a watch, a boat) or a feminine nouns starting with /a/ tonic (a soul, an eagle).

In other cases you should use An or a depending on noun to which the number refers. This rule also applies to all composite figures: twenty-one people, twenty-one hours, twenty-one children, twenty-one users.

When the word thousand It is located between the number and the noun, gender agreement is optional. Based on this, it is normal to say, for example, “twenty-one thousand people” or “twenty-one thousand people.”

These figures are unchanging in number. Therefore, it is not appropriate to say “it is twenty-one o’clock”, but rather “it is twenty-one o’clock”, as noted above.

Do you say ‘what time is it’ or ‘what time is it’?

The civilized way to express the question of the hour is in singular: “What time is it?”. This formula has the meaning of ‘what time is it’. According to him Pan-Spanish Dictionary of Doubtsthe multiple question (what time is it?) “is permissible, though less recommended, and occurs with some regularity in some American countries, especially colloquially.”

The single answer is used to indicate that it is one o’clock in the morning or one o’clock in the afternoon. In the other cases the answer is in the plural: “It is three o’clock in the afternoon.” “It’s four o’clock in the morning”. (F)

SOURCES:

Pan-Spanish Dictionary of Doubts, New grammar of the Spanish language And Spanish dictionary (electronic versions), from the Royal Spanish Academy and the Association of Academies of the Spanish Language.