Paul in his letter to the Corinthians is quoting Isaiah 49:8 and applying it to his ministry:
2 Cor 6:2 (For he saith, I have heard thee in a time accepted, and in the day of salvation have I succoured thee: behold, now is the accepted time; behold, now is the day of salvation.)
Isa 49:8 Thus saith the LORD, In an acceptable time have I heard thee, and in a day of salvation have I helped thee: and I will preserve thee, and give thee for a covenant of the people, to establish the earth, to cause to inherit the desolate heritages;
2 Cor 6:2 (For he saith, I have heard thee in a time accepted, and in the day of salvation have I succoured thee: behold, now is the accepted time; behold, now is the day of salvation.)
Isa 49:8 Thus saith the LORD, In an acceptable time have I heard thee, and in a day of salvation have I helped thee: and I will preserve thee, and give thee for a covenant of the people, to establish the earth, to cause to inherit the desolate heritages;
1. Thus saith the LORD, In an acceptable time have I heard thee, and in a day of salvation have I helped thee... (applicable from Pentecost to the second coming of Christ).
2. ...and I will preserve thee, and give thee for a covenant of the people, to establish the earth, to cause to inherit the desolate heritages; (applicable from the second coming of Christ to the end of the Millennium and beyond)
Therefore Paul could legitimately apply the first segment to his time, and it is still applicable to Jews during the Church Age. Now is indeed the day of salvation for the whole world.
But we are to take the second segment literally in that Christ will establish the earth in righteousness during the Millennium, with redeemed and restored Israel given "for a covenant of the people", and all the "desolate places" of the earth will be renewed and restored to become fruitful. The prophecy of Ezekiel will throw more light on this.