vajras tu tat-kandharam asu-vegah
krntan samantat parivartamanah
nyapatayat tavad ahar-ganena
yo jyotisam ayane vartra-hatye
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Although the thunderbolt revolved around Vrtrasura’s neck with great speed, separating his head from his body took one complete year — 360 days, the time in which the sun, moon and other luminaries complete a northern and southern journey. Then, at the suitable time for Vrtrasura to be killed, his head fell to the ground.