Skanda Purana 1.4.3.123

1.4.3.123

trayodaśī nabhasye ca kṛṣṇā sā hi kaleḥ smṛtā..
yugādayaḥ smṛtā hyetā dattasyākṣayakārakāḥ.. 5.123 ..

The dark thirteenth day in Nabhasya (Bhādrapada) is remembered as the beginning of the Kali yuga. / These are remembered as the beginnings of the yugas, making gifts imperishable.