have been adopted, at different stages, by the Persian Empire, in Hellenistic astrology, and (via Greek transmission) in Gupta India and Tang China. The Babylonian system was received by the Greeks in the 4th century BCE (notably via Eudoxus of Cnidus). However, the designation of the seven days of the week to the seven planets is an innovation introduced in the time of Augustus. The astrological concept of planetary hours is rather an original innovation of Hellenistic astrology, probably first conceived in the 2nd century BCE. The seven-day week was widely known throughout the Roman Empire by the 1st century CE, along with references to the Jewish Sabbath by Roman authors such as Seneca and Ovid. When the seven-day week came into use in Rome during the early imperial period, it did not immediately replace the older eight-day nundinal system. The nundinal system had probably fallen out of use by the time Emperor Constantine adopted the seven-day week for official use in CE 321, making the Day of the Sun () a legal holiday. Achaemenid period The Zoroastrian calendar follows the Babylonian in relating the 7th, 14th, 21st, and 28th of the month to Ahura Mazda. The forerunner of all modern Zoroastrian calendars is the system used to determine dates in the Persian Empire, adopted from the Babylonian calendar by the 4th century BCE. Frank C. Senn in his book Christian Liturgy: Catholic and Evangelical points to data suggesting evidence of an early continuous use of a seven-day week; referring to the Jews during the Babylonian captivity in the 6th century BCE, after the destruction of the Temple of Solomon. While the seven-day week in Judaism is tied to Creation account in the Book of Genesis in the Hebrew Bible (where God creates the heavens and the earth in six days and rests on the seventh; Genesis 1:1-2:3, in the Book of Exodus, the fourth of the Ten Commandments is to rest on the seventh day, Shabbat, which can be seen as implying a socially instituted seven-day week), it is not clear whether the Genesis narrative predates the Babylonian captivity of the Jews in the 6th century BCE. At least since the Second Temple period under Persian rule, Judaism relied on the seven-day cycle of recurring Sabbaths. Tablets from the Achaemenid period indicate that the lunation of 29 or 30 days basically contained three seven-day weeks, and a final week