By the term "harvest" here used is meant the gathering of the remnant of the Jews to Christ.
Jesus' statement plainly is that the gospel age will end with a harvest, during which time he would be present, directing the work of that harvest.

In the earth three and a half years from the time of his consecration and baptism, Jesus was preparing the Jews for the harvest of that age.

We should expect to find a parallel of this with reference to the harvest of the gospel age, and we do find it.

Counting three and a half years from 1874, the time of his presence, brings us to 1878.
During the presence of the Lord from 1874 to 1878 he was making preparation for the harvest of the gospel age.

The Jewish harvest covered a period of forty years, ending in A.D.73.