Acts 18:18

And Paul, having tarried after this yet many days, took his leave of the brethren, and sailed thence for Syria, and with him Priscilla and Aquila: having shorn his head in Cenchraea; for he had a vow.

Yet many days ... The uncertainty of reckoning this in the eighteen months (Acts 18:11) has already been noted. It would seem that from the use of "but" at the beginning of Acts 18:12, the scene before Gallio was introduced retrospectively; and in that case, the "many days" of this place would be the time remaining in the eighteen months.

Shorn his head; for he had a vow ... It is grammatically possible to refer this to Aquila; but the subject of the whole paragraph appears to be Paul, and scholars are sharply divided on the question of whose vow it was and whose head was shaved. In the light of Acts 21:26, it is clear that Paul, as a Jew and not as a Christian, might easily have done such a thing; and if he did not do so, no point could be made of it, because his friend Aquila, who was also a Christian, would hardly have done such a thing against Paul's wishes. Rather than citing extensive arguments with reference to who did it, we are content to say with H. Leo Boles that we simply do not know. As Boles said:

As a Jew, Paul kept up his observance of the ceremonial law for some instances, but refused to impose it upon Gentiles.[24]