Bhagavatapurana 3.11.12

3.11.12

ayane cāhanī prāhur vatsaro dvādaśa smṛtaḥ /
saṃvatsara-śataṃ n-ṇāṃ paramāyur nirūpitam //

12. The two ayanas are the day aud night of the gods in swarga and they are said to make a year comprising twelve months. A hundred years form the full period of human life as observed by the wise.