The Jubilee formula is used here to create a fixed Jubilee framework of 120 Jubilees. The beginning and end dates for the framework are established by "anchoring" the framework on the birth of Jesus in 2 BCE. The assumption is made that this birth  was at the beginning of the 80th Jubilee of 120 Jubilees.

The number of Jubilees - 120 is based on the significance of 120 in the bible. Gen 6:3 shows that man's years would  be 120. Clearly that was not in reference to life expectency and is regarded by many as a probation period. Coincidentally, 120 Jubilees, anchored on Jesus birth, provides a start date for the Jubilee framework of 3873 BCE. 130 years prior to that brings us to 4003 BCE the year that Adam begins. 4004 BCE is the date for Adam's creation given  by Bishop James Ussher. 3873 BCE could also be viewed as the year Seth was born, that is, 130 years afer Adam began (Gen 5:3). In effect the 120 Jubilees running from this date would be the period of God's dealing with men, Adam having been a son of God. This is clear in considering the geneologies set out in Luke 3:23-38