PURPORT

Hiranyakasipu's determination was to occupy the post of Lord Brahma, but this was impossible because Brahma has a long duration of life. As confirmed in Bhagavad-gita (8.17), sahasra-yuga-paryantam ahar yad brahmano viduh: one thousand yugas equals one day of Brahma. The duration of Brahma's life is extremely great, and consequently it was impossible for Hiranyakasipu to occupy that post. Nonetheless, his decision was that since the self (atma) and time are both eternal, if he could not occupy that post in one lifetime he would continue to execute austerities life after life so that sometime he would be able to do so.