觀無量壽佛經義疏 2.10

2.10

'At one time': the times when the Tathāgata preached sūtras were immeasurable and cannot be definitively designated; therefore it says 'at one time.' Furthermore, the Western Land and this land have different systems for calculating years, months, days, and hours; if disseminated to other lands, people would find it hard to comprehend. Furthermore, some sūtras could not be concluded in a single session and were only woven into text later at the compilation; the before and after being varied, it is difficult to designate. For instance, in the present sūtra, King Ajātaśatru committed rebellion, the Tathāgata preached the sūtra, and Ānanda later reiterated it—how could these be at the same time? Therefore, with a single phrase, it encompasses them all completely. Fourth, the lord who preached the teaching.