Hillel II—Hillel ha-Nasi הלל הנשיא
the patriarch to whom tradition ascribes the fixed calendar
Precision is needed here, because the matter is weighty. The tradition itself appears first not among contemporaries but in a responsum of Rav Hai Gaon written in 992—more than six centuries later; it came down through Abraham bar Hiyya's Sefer ha-Ibbur of 1123. There too the year is named: 670 of the Seleucid era, that is 4119 from Creation, 358/9 of the common reckoning.
What exactly Hillel did the sources do not agree. By one reading Rav Hai ascribed to him only the nineteen-year cycle of leap years and not the whole order of reckoning; a letter from the Cairo Geniza of 835/6 sets the festivals on days other than the present calendar gives—so the reckoning was not yet finally fixed by then. We give the tradition as tradition and do not pass it off as a document.
The emperor Julian wrote him a friendly letter and announced that he was burning the registers of the Jewish tax. In halakhah two sayings of his survive, both in the Jerusalem Talmud.
On the timeline
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Sources
Rishonim
- The responsum of Rav Hai Gaon, 992, through Abraham bar Hiyya's Sefer ha-Ibbur, 1123 — the first notice that Hillel fixed the calendar, and the year 670 of the Seleucid era
Scholarship
- The Jewish Encyclopedia (1906) — the years of the patriarchate, Julian's letter, and the cautions about the tradition
Basis: Talmud and midrash.