Is it allowed to say "Quelle heure est-on" instead of "Quelle heure est-il"? What's the difference between these questions?
Quelle heure est-t-il ?
is what is taught at school but is somewhat formal and relatively infrequent.
Quelle heure il est ?
is by far the most common way to ask "what time is it?" in spoken French.
A common but regional (Switzerland and large South-Western France) sentence is:
and you might even hear the colloquial and dubious:
On est quelle heure ?
perhaps groaned by someone not fully awoken in the middle of the night, and due to the proximity and confusion with:
On est quel jour ?
On the other hand, the suggested:
*Quelle heure est-on ?
is extremely improbable from French native speakers if only because its formal structure doesn't mix with the informal and offbeat on.
The fact is there is no real flexibility with these kind of sentences. You can't say either:
il est quel jour ?or il est quelle année ?.
I suspect the specificity of il est vs on est might be due to the fact that while we are fully and clearly "into" the space-time of the days, month and years (on est vendredi, on est en janvier, on est en 2018, on est en hiver...), the ephemeral hour was, until a not so distant past, an imprecise and often inaccurate concept. That means we were not really sure to be "at" a specific hour, but only aware that some device like a sundial was suggesting an hour.
We still say:
il est quelle heure à ta montre ?
meaning that is not necessarily the right information.
TL;DR : It is ok, but why would you?
It depends of what you mean by "allowed".
Is it grammatically correct? Yes. Will that carry the meaning? Sure. But if I hear someone say that, i would assume he's either from Canada or a remote place in France.
It sounds like "Quelle heure fait-il?" for me: it's understandable, yet not the "classic and corect way" of saying it.
That would be correct in my opinion. But that's really odd to the ear. I wouldn't use it myself, as a native speaker, and would be confused for a millisecond if anyone were to ask me that way.
Now if you really think about it, and had to use a more uncommon sentence construction, such as:
Est-il tard ?
I don't think you'd be able to replace it with "on". That wouldn't work, on pronunciation flow side. Or at the very least would sound odd.