Paul expressed the confidence that Christian growth would take place “until the day of Christ Jesus.” He glanced backward to their salvation and forward to the completion of their character when the Lord returns. No doubt the reference to the “day of Christ Jesus” is the “day of the Lord” so common in the Old Testament (Joel 2:1; Amos 5:20). The question is why the end times were included at this point. Although Paul could have thought in terms of the imminent coming of the Lord, he also was more aware of a delay than earlier in his ministry. He wrote of a delay in the Lord’s return as early as 2 Thess 2:1–12, so by the time of the writing of Philippians this delay had been reinforced in his mind. Paul’s use of the phrase “until the day” actually called to mind the consummation of the present age. It was Paul’s way of making two emphases: sanctification was an ongoing process and the process would continue to the end of the age. At that time the believers would be complete in character. They needed not to fear the judgment which characterized that day.
Melick, R. R. (1991). Philippians, Colossians, Philemon (Vol. 32, p. 59). Nashville: Broadman & Holman Publishers.