Those who scientifically understand the past declare that on the very day that Lord Śrī Kṛṣṇa departed for the spiritual world, the influence of the Age of Kali began.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
यस्मिन् कृष्णो दिवं यातस्तस्मिन्नेव तदाहनि । प्रतिपन्नं कलियुगमिति प्राहुः पुराविदः
yasmin kṛṣṇo divaṃ yātastasminneva tadāhani | pratipannaṃ kaliyugamiti prāhuḥ purāvidaḥ
SB 12.2.33