And all the nations will be gathered before him, and he will separate people one from another, just as a shepherd separates the sheep from the goats.

The gathering of all the nations before him means that Jesus acts toward the nations as his prospective flock, figuratively speaking. It is a flock made up of sheep and goats. While it may take part of a day to separate the sheep from the goats in a literal flock, the earth-wide separating of people who are free moral agents takes much longer. This is because the separation is based on each individual’s course of action.