法華義疏 1.121

1.121

「一時」者，謂說經有時，故可信也。今略明四義：一者、佛五十年說法蓋是多時，今對彼多時明此經是一時之說，故云一時也。二者、一謂一部經也。蓋是說一部經時，故云一時也。三者、眾生心行可一之時，即是一乘機發、說一乘教時，故云一時。

"At one time" means that there was a time when the sūtra was preached, and so it can be trusted. Now, four meanings are briefly clarified:
First, the Buddha's preaching the Dharma for fifty years was over a great deal of time; now, contrasting with that great deal of time, it is clarified that this sūtra was preached at one time, hence it says "at one time."
Second, "one" refers to one entire sūtra; it was the time when one entire sūtra was preached, hence it says "at one time."
Third, when the mental activities of sentient beings could be unified—that is, when the capacity for the ekayāna was triggered and the ekayāna teaching was preached—it is called "at one time."