The ancients were astrologically aware that it took about 25920 years for the fixed stars to make one complete rotation, designated as the time period of one Great Year. Divide this number of years by 12 to get the total for a Great Month of 2160 years. The Great Month can be further sub-divided into 30 Great Days of 72 years. Each of the 12 zodiacal signs or constellations could then be considered as Great Months in their duration.
Moses seems to have been aware of this number; if the 7 days of Creation are made to symbolize 7 Great Days, then the total number of years is 72 x 7 = 504 years. Adding the time up from Ha-Adam (a separate creation from the rest of mankind in the Hebrew original) to the Great Flood is 1656 years. Divide this number by 72 and you obtain 23 more Great Days to add to the other seven for a Great Month.
504 + 1656 = 2160 years
The Passover in Exodus is supposed to have occurred under the sign of Aries, the ram. By the time Jesus was born the sign had changed to Pisces, the fish.
The ancients had no way of knowing that the number of years for a Great Month is actually closer to 2155 years.
A further consideration:
The 72 years are in sidereal time as opposed to solar time. This is based on the circular motion of the constellations in the night sky.
One can calculate sideral time with an understanding of a Great Day. Every 72 years one additional day has to be added to the calendar.