In the context, Paul had been talking about all of the groups of people who had seen Jesus after he was resurrected (1 Corinthians 15:3-7). These all seem to have been in the 40 days after Jesus was raised before he was taken into heaven. However, Paul knew that Jesus had only appeared to him long after that time, while he was in the process of persecuting Jesus’ followers. So I think all Paul means is that the time of Jesus appearing to him was out of place, since it was a much later time than when everyone else had seen Jesus. It was also a very surprising and unexpected appearance, very much like an unexpected birth (NET). I think he thought it would have been much better if Jesus had appeared to him along with all the others before he went up into heaven, so that he would not have persecuted the church of God.