SB 7.15.69
aham purabhavam kascid
gandharva upabarhanah
namnatite maha-kalpe
gandharvanam susammatah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Long, long ago, in another maha-kalpa [millennium of Brahma], I existed as the Gandharva known as Upabarhana. I was very respected by the other Gandharvas.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Srila Narada Muni is giving a practical example from his past life. Formerly, during the previous lifetime of Lord Brahma, Narada Muni was one of the denizens of Gandharvaloka, but unfortunately, as will be explained, he fell from his exalted position in Gandharvaloka, where the inhabitants are extremely beautiful and expert in singing, to become a sudra. Nonetheless, because of his association with devotees, he became more fortunate than he was in Gandharvaloka. Even though cursed by the prajapatis to become a sudra, in his next life he became the son of Lord Brahma.
The word maha-kalpe is described by Srila Madhvacarya as atita-brahma-kalpe. Brahma dies at the end of a life of many millions of years. The day of Brahma is described in Bhagavad-gita (8.17):
sahasra-yuga-paryantam
ahar yad brahmano viduh
ratrim yuga-sahasrantam
te ’ho-ratra-vido janah
“By human calculation, a thousand ages taken together is the duration of Brahma’s one day. And such also is the duration of his night.” Bhagavan Sri Krsna can remember incidents from millions of years ago. Similarly, His pure devotee like Narada Muni can also remember incidents from a past life millions and millions of years ago.