The words translated as “new creature” in verse 17 carries the meaning of something that is not “new” in time, but “new” in form or quality.
It implies a new nature quite different from anything previously existing.[i]
What Paul is stating, is that the one who is “in Christ” has literally entered a whole new type of life, a life that is “hid with Christ in God”.
For ye are dead, and your life is hid with Christ in God.
Paul lived in the present reality that on the Damascus road Saul was replaced by Paul. He was given not just a new lease on life; he was given a new life. Paul basked in the truth that every day in Christ he was a new him.
[i] Jamieson, R., Fausset, A. R., & Brown, D. (1997). Commentary Critical and Explanatory on the Whole Bible (Vol. 2, p. 309). Oak Harbor, WA: Logos Research Systems, Inc.