The following is taken from PEU1 123.5:
Could have + past participle can refer to present situations which were possible but have not been realised.
He could have been Prime Minister now if he hadn't decided to leave politics.
We could have spent today at the seaside, but we thought it was going to rain, so we decided not to.
The above two are the examples of past irrealis conditionals.
But I just don't understand why "now" / "today" agree with the past tense (could have been Prime Minister now / could have spent today).
I would think they should be like this:
He could be Prime Minister now if he hadn't decided to leave politics.
We could spend / be spending today at the seaside, but we thought it was going to rain, so we decided not to.
Do these two alternatives make sense? How do they differ from the original ones?
Interestingly, as @Fantasier suggested, PEU 259.3 also introduced a similar usage:
We sometimes use structures with would have ... to talk about present and future situations which are no longer possible because of the way things have turned out.
It would have been nice to go to Australia this winter, but there's no way we can do it. (OR It would be nice ...)
If my mother hadn't knocked my father off his bicycle thirty years ago, I wouldn't have been here now. (OR ... I wouldn't be here now.)
But PEU failed to provide more explanations about why this is acceptable and how native speakers think of and use it.
1. PEU = Michael Swan's, Practical English Usage.