大般涅槃經義記 1.37

1.37

問曰：何故此時涅槃？下經自釋，依《阿含經》「如來八月八日涅槃。」此經下文亦有此相，故下文言「須跋陀羅竟安居已，當至我所，是故我昔告魔波旬：『過三月已，吾當涅槃。』」今言二月十五日者，當是見聞不同故爾。

Question: Why did he enter nirvāṇa at this time? The sūtra below explains it itself. According to the Āgamas, “The Tathāgata entered nirvāṇa on the eighth day of the eighth month.” The subsequent text of this sūtra also has this feature, so the text below says: “After Subhadra finishes his retreat, he will come to my place; therefore I formerly told Māra Pāpīyas: ‘After three months have passed, I shall enter nirvāṇa.’” The current statement of the fifteenth day of the second month must be due to differences in what was seen and heard.