The Rif on Sefirat haOmer

Soon to be posted on the Rif blog. At the end of hilchot Pesach, he cites the relevant gemara in Menachot about sefirat haOmer. In Pesachim, page 27b in the Rif to 28a:

{Menachot 65b}
And we are obligated to count the days of the seven weeks, from the evening of the 15th of Nissan which is the night of {=beginning} the 16th, for it is written {Vayikra 23:15}:

טו וּסְפַרְתֶּם לָכֶם, מִמָּחֳרַת הַשַּׁבָּת, מִיּוֹם הֲבִיאֲכֶם, אֶת-עֹמֶר הַתְּנוּפָה: שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה.

15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;

And the Sages learnt {in a brayta}: {Vayikra 23:15}:

טו וּסְפַרְתֶּם לָכֶם, מִמָּחֳרַת הַשַּׁבָּת, מִיּוֹם הֲבִיאֲכֶם, אֶת-עֹמֶר הַתְּנוּפָה: שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה.

15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;

Each and every person counts for himself.
מִמָּחֳרַת הַשַּׁבָּת - "from the morrow after the day of rest" - from the morrow of Yom Tov {of Pesach}

{Menachot 66a}
And a brayta also says so: {from same pasuk:}

טו וּסְפַרְתֶּם לָכֶם, מִמָּחֳרַת הַשַּׁבָּת, מִיּוֹם הֲבִיאֲכֶם, אֶת-עֹמֶר הַתְּנוּפָה: שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה.	15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;
טז עַד מִמָּחֳרַת הַשַּׁבָּת הַשְּׁבִיעִת, תִּסְפְּרוּ חֲמִשִּׁים יוֹם; וְהִקְרַבְתֶּם מִנְחָה חֲדָשָׁה, לַיהוָה.	16 even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the LORD.

Perhaps this means that he should reap and bring {as an offering}, and then begin counting whenever he likes? Therefore it teaches {Devarim 16:7}:

ט שִׁבְעָה שָׁבֻעֹת, תִּסְפָּר-לָךְ: מֵהָחֵל חֶרְמֵשׁ, בַּקָּמָה, תָּחֵל לִסְפֹּר, שִׁבְעָה שָׁבֻעוֹת.

9 Seven weeks shalt thou number unto thee; from the time the sickle is first put to the standing corn shalt thou begin to number seven weeks.

{Menachot 66a continues}
Perhaps he should reap, count, and then bring {as an offering}? Therefore it teaches {Vayikra 23:15-16}:

טו וּסְפַרְתֶּם לָכֶם, מִמָּחֳרַת הַשַּׁבָּת, מִיּוֹם הֲבִיאֲכֶם, אֶת-עֹמֶר הַתְּנוּפָה: שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה.	15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;
טז עַד מִמָּחֳרַת הַשַּׁבָּת הַשְּׁבִיעִת, תִּסְפְּרוּ חֲמִשִּׁים יוֹם; וְהִקְרַבְתֶּם מִנְחָה חֲדָשָׁה, לַיהוָה.	16 even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the LORD.

Perhaps he should reap by day and bring by day and count by day? Therefore it teaches us {Vayikra 23:15}:

טו וּסְפַרְתֶּם לָכֶם, מִמָּחֳרַת הַשַּׁבָּת, מִיּוֹם הֲבִיאֲכֶם, אֶת-עֹמֶר הַתְּנוּפָה: שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה.

15 And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;

When are they complete? When he begins in the evening. How so? Reaping and counting at night, and bringing {as an offering} by day.

And just as it is a commandment to count days, so is it a commandment to count weeks {as both days and weeks are mentioned in the pesukim}. For Abaye said: It is a commandment to count days and a commandment to count weeks.

And it is forbidden for us to eat chadash {the new crop} until the evening of the 17th of Nissan which is the night of the 18th, for it is written {Vayikra 23:14}:

יד וְלֶחֶם וְקָלִי וְכַרְמֶל לֹא תֹאכְלוּ, עַד-עֶצֶם הַיּוֹם הַזֶּה--עַד הֲבִיאֲכֶם, אֶת-קָרְבַּן אֱלֹהֵיכֶם: חֻקַּת עוֹלָם לְדֹרֹתֵיכֶם, בְּכֹל מֹשְׁבֹתֵיכֶם. {ס}

14 And ye shall eat neither bread, nor parched corn, nor fresh ears, until this selfsame day, until ye have brought the offering of your God; it is a statute for ever throughout your generations in all your dwellings. {S}

And they learn {in a Mishna in the third perek of Orlah}: The chadash {new crop} is forbidden Biblically.

And we say in the gemara {Menachot 68b}: Ravina said: They said to me: If your father ate {chadash} on the evening of 17th which was the night of the 18th, he holds like Rabbi Yehuda, who said: Is it not forbidden Biblically, for it is written {Vayikra 23:14}:

יד וְלֶחֶם וְקָלִי וְכַרְמֶל לֹא תֹאכְלוּ, עַד-עֶצֶם הַיּוֹם הַזֶּה--עַד הֲבִיאֲכֶם, אֶת-קָרְבַּן אֱלֹהֵיכֶם: חֻקַּת עוֹלָם לְדֹרֹתֵיכֶם, בְּכֹל מֹשְׁבֹתֵיכֶם. {ס}

14 And ye shall eat neither bread, nor parched corn, nor fresh ears, until this selfsame day, until ye have brought the offering of your God; it is a statute for ever throughout your generations in all your dwellings. {S}

until {ad} the actual {itzumo} day, and he holds that until {ad} is inclusive {ad veAd bekhlal -- thus that day would also be forbidden}, and we are concerned for safek {doubt - in this case sefeika deyoma}. Therefore, the 17th which is in doubt of being the 16th, one should not eat until night.
And so is the halacha.