In the Lotus Sūtra proper, … the Buddha revealed the single path to Enlightenment, having synthesized three kinds of teaching (kaisan ken’itsu). He declared in the theoretical (shakumon) section of the Lotus Sūtra: “Only Buddhas wholly perceive the reality of all phenomena; truth will be revealed after preaching provisional teachings for more than forty years;” and “He will discard provisional teachings and will concentrate only on the supreme way.” The Buddha of Many Treasures then declared that those words of the Buddha in the eight (from the second to ninth) chapters in the theoretical section (shakumon) of the Lotus Sūtra were all true. What then should have been left unrevealed? Nevertheless, the eternal life of the Buddha was not revealed; instead it is said, “At first, He sat at the place of enlightenment, gazed on the tree, and walked around it meditating.” This is the greatest wonder of all.
Thus it is stated in the fifteenth “Appearance of Bodhisattvas from Underground” chapter of the Lotus Sūtra that Bodhisattva Maitreya wondered why the Buddha claimed to have taught the great bodhisattvas, who had never been seen before in the last forty years or so, and caused them to aspire for enlightenment. So, he asked: “When You, the Buddha, were the crown prince, You left the palace of the Śākya clan, sat in meditation under the bodhi tree not far from the city of Gayā and obtained the perfect enlightenment. It has only been some forty years since then. How could You, World Honored One, have achieved so much in a short period of time?”
It was at this point that Lord Śākyamuni Buddha decided to preach “The Life Span of the Buddha” chapter in order to dispel such doubts. Referring to what has been said in the pre-Lotus sūtras and in the theoretical section of the Lotus Sūtra, He said, “Gods, men and asura demons in all the worlds think that I, Śākyamuni Buddha, left the palace of Śākya clan, sat under the bodhi tree not far from the city of Gayā, and attained perfect enlightenment.” Then He answered the question squarely by declaring, “To tell the truth, however, it has been innumerable and incalculable kalpa since I attained Buddhahood.”
Kaimoku-shō, Open Your Eyes to the Lotus Teaching, Writings of Nichiren Shōnin, Doctrine 2, Page 47