Gemara

9b - Beraisa: If one plants (a tree), is Mavrich (inserts branches into the ground to take root) or grafts 30 days before Rosh Hashanah of Shemitah, this counts like a year, and one may keep the tree in Shemitah;

If it was less than 30 days this does not count like a year, and he may not keep it in Shemitah. The fruits of this tree are forbidden until Tu bi'Shvat, regarding Orlah (three years later) and Revai (the year after that).

Question: What is the source of this?

Answer (R. Chiya bar Aba): "Uv'Shanah ha'Revi'is uv'Shanah ha'Chamishis" - sometimes Orlah still applies in the fourth year and Revai in the fifth.

Rishonim

Rif and Rosh (1:2): The Beraisa is like R. Meir, who requires 30 days to take root and says that one day in the year is like a year. The Halachah follows Rav in Isurim, and he rules like R. Yosi, who requires 14 days to take root and 30 days to count like a year.

Rambam (Hilchos Neta Revai 9:8): Tishrei 1 is Rosh Hashanah for Orlah and Revai. These are counted from the time of planting. We do not count from Rosh Hashanah to Rosh Hashanah. Rather, 30 days in a year are considered a year, if it took root 30 days before Rosh Hashanah. For trees, two weeks are needed to take root.

Rambam (9,10): Therefore, if one plants 44 days before Rosh Hashanah, this counts like a year. Still, the Isurim of Orlah and Revai do not cease until Tu bi'Shvat. E.g. if one planted on the 15th of Av in the 10th year of Yovel, the fruits are Orlah until Tu bi'Shvat of the 13th year. All growth during this period is Orlah, even if it was not completed until later. All growth from Tu bi'Shvat of the 13th year until Tu bi'Shvat of the 14th year is Neta Revai, and it must be redeemed. If one of these years was a leap year, the law (of Orlah or Revai) applies to the added month.

Rambam (11): If one planted on the 16th of Av in the 10th year, it does not count like a year. Therefore, the fruits are Orlah until the end of the 13th year. All growth during the 14th year is Revai.

Source (Ra'avad): This is learned from the Yerushalmi. The Bavli supports this. It says 'the fruits of this tree are forbidden until Tu bi'Shvat, implying that those of other trees are not. Also, it says that sometimes Orlah applies in the fourth year. This connotes this is not always true.

Rebuttal (Ba'al ha'Ma'or): The fruits are always forbidden until Tu bi'Shvat. It says sometimes because it is uncommon for Chanatah (budding) to occur between Rosh Hashanah and Tu bi'Shvat. (The status of the fruits depends on the time of Chanatah.) Since Chanatah is uncommon in this period, it is not common for Orlah or Revai to apply in the fourth or fifth (calendar) year. It says 'the fruits of this tree are forbidden until Tu bi'Shvat, because its planting (less than 30 days before Rosh Hashanah) caused the Isurim of Orlah and Revai to continue an additional year (but in any case, the Isurim end on Tu bi'Shvat). The Yerushalmi supports this.

Ran (DH v'Ha): This is an argument in the Yerushalmi. One opinion requires waiting until Tu bi'Shvat only when we were lenient to consider 30 days like a year. The other opinion is more reasonable. It says that waiting until Tu bi'Shvat is not a stringency. Rather, trees grow on Rov Mayim. I.e., if Chanatah was between Rosh Hashanah and Tu bi'Shvat, it was primarily due to sap from before Rosh Hashanah, therefore it is as if the Chanatah was before Rosh Hashanah. Tishrei 1 is called Rosh Hashanah for trees, because it determines in which year Tu bi'Shvat will permit the fruits (based on whether or not it took root long enough before Tishrei 1).

Rebuttal (Kesef Mishneh): This is not an argument in the Yerushalmi. A question was asked and answered (like the Rambam), and the questioner was silent. It is even possible that the questioner agrees with the Rambam's law!

Gra (YD 294:13): The Rishonim had different texts of the Yerushalmi.

Rambam (12): If one planted between Rosh Hashanah and Tu bi'Shvat, the fruits are Orlah from three years from day to day. All growth during the fourth year from day to day is Revai. I will not answer what I saw the Ge'onim write, for surely it is a textual mistake.

Ra'avad: 'From day to day' means from Tu bi'Shvat until Tu bi'Shvat. The three years are not from the beginning of Tishrei until the beginning of Tishrei, for we must deduct the days from Rosh Hashanah until it was planted and took root.

Ri Korkus: The Ra'avad teaches that whenever one planted between Rosh Hashanah and Tu bi'Shvat, he must wait until Tu bi'Shvat. Perhaps the Rambam agrees. Alternatively, the Rambam holds that we never wait more than three years from day to day. The Rambam did not specify three years from Klitah (when it took root). It seems that he counts from when it was planted. Alternatively, he relies on what he wrote above that Klitah is primary.

Kesef Mishneh: The Rambam did not need to say that the three years do not end on Tishrei 1, for it is obvious (this is not three years from day to day). The Ra'avad is not precise when he says from Tu bi'Shvat until Tu bi'Shvat, for the three are counted after 14 days for Klitah.

Rebuttal (Gra, ibid.): Days of Klitah are needed at the beginning for 30 days to be considered a year, but the Rambam does not require them for years of Orlah. The Toras Kohanim and Yerushalmi support him.

Rosh (Hilchos Orlah (printed after Menachos) Siman 9): Tishrei 1 is Rosh Hashanah for saplings, but Tu bi'Shvat is Rosh Hashanah for (mature) trees. Therefore, the years of Orlah and Revai end on Tu bi'Shvat. Nowadays, people are not careful about Orlah for more than three years. This is because nowadays no tree has Chanatah before Tu bi'Shvat.

Ma'adanei Yom Tov (1): If a tree was planted between Av 16 and Rosh Hashanah, we must wait more than three years, until the fourth Rosh Hashanah! Perhaps the Rosh means that nowadays no tree has Chanatah between Av 16 and Tu bi'Shvat.

Note: The Rosh says that nowadays the only Revai we are concerned for is grapes, because in Chutz la'Aretz we follow the opinion that is most lenient in Eretz Yisrael. Perhaps he means that nowadays no vine has Chanatah before Tu bi'Shvat.

Poskim

Shulchan Aruch (YD 294:4): The three years of Orlah are not counted from day to day. Rather, it is three calendar years, which start in Tishrei. The extra month of a leap year is included in the years of Orlah and Revai. Sometimes the three years are only two years are 44 days, and sometimes they are more than three years. If one planted before Av 16 and there were 44 days before Tishrei 1, when Tishrei 1 comes it is counted like one year. He counts two more years. If one planted on Av 16 or later, he counts three full more years from Tishrei 1. Any fruits for which Chanatah was between Tishrei 1 and Tu bi'Shvat of the fourth year are Orlah, even if they are finished after Tu bi'Shvat. Fruits for which Chanatah is between Tu bi'Shvat of the fourth year and Tu bi'Shvat of the fifth year are Revai. Fruits for which Chanatah is after Tu bi'Shvat of the fifth year are Chulin.

We said that if Chanatah was between Tishrei and Tu bi'Shvat of the fourth year, the fruits are Orlah even if they are finished after Tu bi'Shvat. This is only regarding trees for which we were lenient to consider 44 days like a year. Therefore, we are stringent at the end. Some say that we do not distinguish.

Shach (11): Even if something was planted far more than 44 days before Rosh Hashanah, we were lenient to say that Rosh Hashanah begins the second year, even though it is less than one year after it was planted.