Hm. What does this mean? Paul is stating that he has laid the foundation, according to the grace of God which was given to him. Others are building on the foundation Paul has laid. But the foundation that is laid is built on Jesus Christ. If Paul was tasked by Christ to lay a foundation based on the grace of God that was given to him, why not thank Peter, if their gospels were the same gospel of the grace of GOd?
Moses was the master-builder of the tabernacle. God gave him the plans and specifications for it, and said, "See...that thou make all things according to the pattern showed to thee in the mount" (Heb. 8:5). Moses represented the law.
Paul represents grace. Paul too was a master-builder. To him God committed the plans and specifications for a greater building, "an holy temple", the church which is Christ's body. Step by step, revelation by revelation the details were made known to him by direct revelationS so that, as "a wise (intelligent) master-builder", he had the right and responsibility to outline those details to us, the body of Christ.
"It is not expedient for me doubtless to glory. I will come to visionS and revelationS of the Lord." 2 Cor. 12:1. Visions and revelations, both plural.
How can you expect us to give you a firm date when Paul says that his ministry and commission were based on REVELATIONS, not one, but more. It's a progressive revelation and building of the body of Christ, not a one moment in time event.
Frankly I hear the same reasoning from Mormons and Catholics here on FR.