In the Jewish religion the act of creation is the starting point of year one in the calendar. According to tradition the first work of chronology is the Seder Olam, attributed to the second-century sage Yose ben Halafta. In this work the calculation is based on Biblical genealogical tables, the length of lives recorded in Scripture, and the creation of the world in six days: on this reckoning the year of creation was 3761 bce. The Jewish calendar itself is based on a lunar year of twelve months (of twenty-nine or thirty days). The year is thus approximately 354 days. The shortage of eleven days between lunar and solar years is made up by adding a thirteenth month in certain years. In 356 ce the patriarch Hillel II introduced a permanent calendar based on mathematical and astrological calculations – this calendar has remained in force with only minor modifications.