Rif Pesachim 28a {Menachot 66a continues; 68b}

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28a

{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.