When the constellation of the seven sages is passing through the lunar mansion Maghā, the Age of Kali begins. It comprises twelve hundred years of the demigods.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
यदा देवर्षयः सप्त मघासु विचरन्ति हि । तदा प्रवृत्तस्तु कलिर्द्वादशाब्दशतात्मकः
yadā devarṣayaḥ sapta maghāsu vicaranti hi | tadā pravṛttastu kalirdvādaśābdaśatātmakaḥ
SB 12.2.31