Is this a correct sentence: This building was officially opened by the major of X city by February 12.?

Answer 1

I don't think so, because the phrase "by February 12" is confusing. Depending on what you are trying to get across, the phrase should be "on February 12" or "before February 12." You could also use "previous to February 12."

If you do want to use the word "by", you could the construct the sentences as, "By February 12, this building had officially been opened by the mayor of X city."

Hope that helps.

Answer 2

No. Nothing like one. "Was" is past tense showing that something has already happened so you would have to say "This building WAS officially opened by the mayor of X city ON Feb. 12."
If the event hasn't happened, then the adverb "by" is used to indicate "not later than" so, "This building WILL BE opened by the mayor of X city BY Feb. 12."
It has to be one or the other to be consistent.