I believe what he was referring to was the "precession of the equinox." Ptolemy knew about it (~175 AD) and probably Hipparchus (~150 BC) as well. Like a spinning top, the orientation of the North Pole "precesses" or rotates around a line perpendicular to the plane of the Earth's orbit, completing one cycle in 26,000 years.
Because we care more about the variations in the seasons than in the position of the sun against the background constellations, we reckon seasons from the position of the sun in the sky - high in summer, low in winter. This leads to a solar year of about 365.2422 days. The Gregorian calender approximates the length of the year as 365.2425 days (365 + 1/4 -1/100+1/400). The "sidereal" year or the time it takes the sun to return to the same place against the fixed stars is closer to 356.2564 days. The difference is due to that precession jive.
Although this doesn't seem like a big difference,the Gregorian calendar (or any solar calendar) loses about 1.4 days / century compared to a solar calendar. The Gregorian calendar reform accounted for days lost since the Council of Nicea set fixed holy days in about CCCXXIII - made the date what it would have been had we been using the Gregorian Calendar all those years.
In the intervening 17 centuries, the position of the constellations relative to the "vernal equinox" - the intersection of the ecliptic plane and the equitorial plane in the direction of the ascending node (sun crossing the equator from South to North) has moved about a month. If you go back to the times of the Babylonians even more, compared to a solar calendar.
Astrology has its roots deep in antiquity and the association of star signs with solar calendar dates goes back far before the Council of Nicea. So to the extent that dates are associated with position of the "Sun" - the notion is profoundly geocentric - against the background stars, dates slip 1.4 days per century, but will return to original positions in 26,000 years (~1.4 deg/cen x 260 cent ~ 360 degrees). In about 22,000 years, the stars will be close to where they were in Babylonian times.