Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 33
Sanskrit:
वज्रस्तु तत्कन्धरमाशुवेग:कृन्तन् समन्तात् परिवर्तमान: ।न्यपातयत् तावदहर्गणेनयो ज्योतिषामयने वार्त्रहत्ये ॥ ३३ ॥
ITRANS:
vajras tu tat-kandharam āśu-vegaḥkṛntan samantāt parivartamānaḥnyapātayat tāvad ahar-gaṇenayo jyotiṣām ayane vārtra-hatye
Translation:
Although the thunderbolt revolved around Vṛtrāsura’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 Vṛtrāsura to be killed, his head fell to the ground.