The mitzvah of Shemittah is introduced in the following way in Parshas Behar (Vayikra 25:1-4):
וַיְדַבֵּ֤ר ה' אֶל־מֹשֶׁ֔ה בְּהַ֥ר סִינַ֖י לֵאמֹֽר׃ דַּבֵּ֞ר אֶל־בְּנֵ֤י יִשְׂרָאֵל֙ וְאָמַרְתָּ֣ אֲלֵהֶ֔ם כִּ֤י תָבֹ֙אוּ֙ אֶל־הָאָ֔רֶץ אֲשֶׁ֥ר אֲנִ֖י נֹתֵ֣ן לָכֶ֑ם וְשָׁבְתָ֣ה הָאָ֔רֶץ שַׁבָּ֖ת לַה'׃ שֵׁ֤שׁ שָׁנִים֙ תִּזְרַ֣ע שָׂדֶ֔ךָ וְשֵׁ֥שׁ שָׁנִ֖ים תִּזְמֹ֣ר כַּרְמֶ֑ךָ וְאָסַפְתָּ֖ אֶת־תְּבוּאָתָֽהּ׃ וּבַשָּׁנָ֣ה הַשְּׁבִיעִ֗ת שַׁבַּ֤ת שַׁבָּתוֹן֙ יִהְיֶ֣ה לָאָ֔רֶץ שַׁבָּ֖ת לַה' שָֽׂדְךָ֙ לֹ֣א תִזְרָ֔ע וְכַרְמְךָ֖ לֹ֥א תִזְמֹֽר׃
Hashem spoke to Moshe on Mount Sinai: Speak to the Children of Israel and say to them: When you enter the land that I will give to you, the land shall observe a sabbath of Hashem: Six years you may sow your field and six years you may prune your vineyard and gather in the yield. But in the seventh year the land shall have a sabbath of complete rest, a sabbath of Hashem: you shall not sow your field or prune your vineyard.
So the Torah says expressly that this injunction starts when they enter the land. But when you think about it, this didn't take effect upon them entering the land! In fact it only happened in the 21st year?!
We know from sefer Yehoshua that Bnei Yisrael spent 7 years conquering the land and then another 7 splitting up the land, which would mean by the time they actually got round to keeping Shemittah it was the third round of seven years - in which case how do we understand the clause in Behar which says that it will be kept expressly "when you enter the land"?