Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 6


त्रसरेणुत्रिकं भुङ्क्ते य: काल: स त्रुटि: स्मृत: ।शतभागस्तु वेध: स्यात्तैस्त्रिभिस्तु लव: स्मृत: ॥ ६ ॥


trasareṇu-trikaṁ bhuṅkteyaḥ kālaḥ sa truṭiḥ smṛtaḥśata-bhāgas tu vedhaḥ syāttais tribhis tu lavaḥ smṛtaḥ


The time duration needed for the integration of three trasareṇus is called a truṭi, and one hundred truṭis make one vedha. Three vedhas make one lava.


It is calculated that if a second is divided into 1, 687.5 parts, each part is the duration of a truṭi, which is the time occupied in the integration of eighteen atomic particles. Such a combination of atoms into different bodies creates the calculation of material time. The sun is the central point for calculating all different durations.