Dummy "it"

“When is it a good time to call you?”

If I am correct that the “it” is an expletive or dummy “it” that is holding the place for the real subject, then what is the real subject in that sentence?

Thank you.

I think it’s a dummy “it”, but I wouldn’t say it was “holding the place for the real subject”. I think it’s more satisfying a grammatical requirement in a situation where there is no real subject.

A dummy subject is used when there is no real one, I suppose.

It snows in May.
There was only one man present there.

Thank you, Dozy and T.H., for your replies.

Dozy:

In “It is difficult to learn English,” “everybody” agrees that “it” is the dummy subject for “to learn English.” That is: To learn English + is + difficult.

Well, I was wondering about that “it” in my first post.

At another helpline, someone (quite confidently) opined that the “it” referred to “good time to call you.” So I was wondering what others thought, for I do not feel 100% comfortable with his suggestion.

Thank you.

I don’t agree with either of those theories. Substitution would give:

“To learn English is difficult to learn English.”
“When is a good time to call you a good time to call you?”

The first is nonsensical. The second is a strange question, like a riddle, which does not have the intended meaning.

Thank you, Dozy.