Gittin 54A

ואין מונין לשבתות.

But they do not count from Shabbat, as there is no significance to the day of the week on which the tree was planted, and consequently no one remembers that the tree was planted on Shabbat.

דבר אחר: נחשדו ישראל על השביעית, ולא נחשדו על השבתות.

Alternatively, the difference between Shabbat and the Sabbatical Year can be explained as follows: Jews are suspected of desecrating the Sabbatical Year, but they are not suspected of desecrating Shabbat. Therefore, R' Meir imposes a penalty for the unintentional desecration of the Sabbatical Year, but he imposes no such penalty for the unintentional desecration of Shabbat.

מאי ״דבר אחר״? הכי קאמר: וכי תימא שבת נמי, זימנין דמיקלע יום שלשים ⁠– בשבת, דאי נטע ההוא יומא הוא דסלקא ליה שתא, ואי לא ⁠– לא סלקא ליה שתא,

The Talmud asks: What is the point of R' Meir adding the second reason, introduced with the term alternatively? The Talmud answers: This is what R' Meir is saying: And if you would say to contest the first reason: Jews also count from Shabbat, and unless the tree is uprooted, people will remember that it was planted on Shabbat, as at times the 30th day before Rosh HaShana falls on Shabbat. As, if he planted the tree on that day, the time from when he planted it counts for him as a full year with regard to the prohibition of fruit that grows during the first 3 years after the tree was planted. And if he did not plant it on that Shabbat, but rather on the next day, the 29th day before Rosh HaShana, it does not count for him as a full year. In such a case people will remember that the tree was planted on Shabbat, and they might come to think that planting a tree on Shabbat is permitted.