瑜伽論記 11.243

11.243

三藏兩解。一云，未來本無以當有故故名無常，過去亦無以曾有故故名無常。第二解云：當有名未來，正有之時當有無故名無常，還無名過去，正有之時還無故名過去無常。

The Tripiṭaka Master gives two explanations. One says: The future is fundamentally non-existent, but because it will exist, it is called impermanent; the past is also non-existent, but because it previously existed, it is called impermanent. The second explanation says: 'Will exist' is called the future; at the very time of its existence, because the future existence ceases to be, it is called impermanent. 'Returning to non-existence' is called the past; at the very time of its existence, because it returns to non-existence, it is called past impermanence.