Complex sentence: The new highway in the towns of Pitcher and Pharsalia was completed in July 1984, at which time NY 23 was rerouted to follow the highway and the entirety of the pre-approved highway maintenance swap went into effect.
Split into simpler sentences: The new highway progressing through the towns of Pitcher and Pharsalia was completed in July 1984. At which time NY 23 was rerouted to follow the highway and the entirety of the pre-approved highway maintenance swap went into effect.