The Shulhan 'Aruch (OC 489:10) states:
אסור לאכול חדש אף בזמן הזה בין לחם בין קלי בין כרמל עד תחלת ליל יח בניסן ובארץ ישראל עד תחלת ליל יז בניסן
"It is forbidden to eat new (grains) even today ... until the start of the night of 18 Nisan, and in the Land of Israel until the start of the night of 17 Nisan."
Why the difference, and what is the source?
(Background: Biblically, the new grain becomes permitted at dawn on the 16th of Nissan if there is no Korban Omer being brought, but Rabbi Yochanon ben Zakai enacted to wait the whole day now that there is no Temple.)
The Talmud (Menachot 68b) records:
רב פפא ורב הונא בריה דרב יהושע אכלי חדש באורתא דשיתסר נגהי שבסר קסברי חדש בחוצה לארץ דרבנן ולספיקא לא חיישינן ורבנן דבי רב אשי אכלו בצפרא דשבסר קסברי חדש בחוצה לארץ דאורייתא ורבן יוחנן בן זכאי מדרבנן קאמר וכי תקין ליום הנף לספיקא לא תקין אמר רבינא אמרה לי אם אבוך לא הוה אכיל חדש אלא באורתא דשבסר נגהי תמניסר דסבר לה כר' יהודה וחייש לספיקא
R. Papa and R. Huna the son of R. Joshua used to eat the new corn on the night of the sixteenth day which is really the beginning of the seventeenth day, for they hold the view that the prohibition of the new corn outside the land [of Israel] is only Rabbinical and that the doubt need not be taken into account. The Rabbis of the school of R. Ashi used to eat it on the morning of the seventeenth, for they hold that the prohibition of the new corn outside the land of Israel is Biblical, but that the ruling of R. Johanan b. Zakkai was only a Rabbinic ordinance; and this ordinance, they maintain, was intended to apply only to the actual day of the waving but not to the day of doubt. Rabina said, ‘My mother told me that your father did not eat of the new corn until the night of the seventeenth which is the beginning of the eighteenth, for he is of the same opinion as R. Judah and also takes into account the day of doubt’. (Translation from Soncino)
The Rambam (M"A 10:2), Rif (28a) and Rosh (42) all rule like Ravina (that the new grain in the Diaspora is biblically forbidden and RYBZ's enactment applies to the second doubtful day as well) who is recorded last, and so rules the Shulchan Aruch.
