Cumont dated Thessalos' treatise to the middle of the first century ce on the basis of its dates for the sun's ingress into the signs of the zodiac. Since astrologers and astronomers in the early Roman period used sidereal rather than tropical longitudes in marking the positions of the sun, the moon, and the planets, the date at which the sun entered a particular sign was delayed by about a day every 100 years, according to ancient estimates. This allowed Cumont to estimate when the treatise was composed by comparing Thessalos' dates to a calendar of solar ingresses interpolated from information found in Varro's De re rustica, which was composed in 37 bce. The basic principle is sound, but Cumont's dating can now be checked and adjusted using a much larger volume of astronomical data that has been gathered in subsequent studies of ancient astronomy and astrology.

In comparing modern calculations with those of twenty-eight horoscopes recorded in Vettius Valens dating from 37 to 188 ce, Neugebauer and Van Hoesen found that calculations of the sun's longitude by the second-century ce astrologer were greater than modern ones by 2 to 7 degrees, and that the differences tended to decrease in the horoscopes calculated for later dates. This decreasing trend, they observed, was roughly commensurate with the precession of the equinoxes. They concluded that these calculations were based on tables which differed from modern calculations by ca. +5° in 50 ce and ca. +3.5° in 160 ce, figures which were comparable to the deviations observed in the Demotic and Greek planetary tables preserved on papyrus.