When we are not sure about the past situation, how do we use if-clause?

I'm not sure whether he came to London last night. If he did, today will be a perfect day.

I'm not sure whether he has come to London last night. If he has, today will be a perfect day.

Which one is correct?

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    They're both "syntactically valid" - but idiomatically, the second version is incredibly unlikely. You should avoid using the Perfect construction (has come instead of came) as a general principle in any context where you're not sure whether you need it or not. – FumbleFingers Aug 28 '20 at 14:27
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    last night, month, year, yesterday, two days ago, etc. trigger the simple past. – Lambie Aug 28 '20 at 14:58

Change your sentence slightly (while keeping the tense) to make it more idiomatic.

A: I am not sure whether he arrived in London last night.
B: If he has (arrived), today will be perfect.

Use of the present perfect perfectly fits the situation.

B is assuming that if he arrived last night, he will still be there - and presumably they can meet up.

It would also be possible to respond:

B: If he did (arrive), today will be perfect.

using the auxiliary did with the base form of the verb arrive.

The difference is that conditional response if he did arrive does not assume that he is still there although this is still implied.

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