Yuga-Vibhāga and Kāla-Pramāṇa
Measures of Time and the Four Yugas
नव यानि सहस्राणि वर्षाणां मानुषाणि तु / अन्यानि नवतिश्चैव ध्रुवः संवत्सरः स्मृतः
nava yāni sahasrāṇi varṣāṇāṃ mānuṣāṇi tu / anyāni navatiścaiva dhruvaḥ saṃvatsaraḥ smṛtaḥ
Nine thousand human years, and a further ninety years besides—this is remembered as the fixed (Dhruva) saṃvatsara.