ANSWER: The fact is more important than the date. This fact is implied in all his exhortations and prayers, for he would not have urged others to obtain what he himself had not received. Nor would he have appealed to the Searcher of hearts as he did in 1 Thess. 2:10, to witness "how holily" he was living. In Gal. 1:15 there is noted a crisis in his experience after his conversion, which may have been his entire sanctification: "But when [it] was the good pleasure of God, who separated me (unto the Gospel Rom. 1:1) from my birth, and called (regenerated) me through his grace, to reveal His Son in me," etc. This is not the revelation of Christ in his natural vision, but rather the inward manifestation of Christ to his spiritual perception quickened into life when he was born from above. No unregenerate man can have an inward revelation of Christ. This may be Paul's phrase for his entire sanctification. It corresponds with modern experiences of this grace as the writer can testify.
— From Steele's Answers pp. 17,18.