SB 3.11.6
trasarenu-trikam bhunkte
yah kalah sa trutih smrtah
sata-bhagas tu vedhah syat
tais tribhis tu lavah smrtah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The time duration needed for the integration of three trasarenus is called a truti, and one hundred trutis make one vedha. Three vedhas make one lava.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is calculated that if a second is divided into 1, 687.5 parts, each part is the duration of a truti, 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.