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SB 3.11.6

trasareṇu-trikaṁ bhuṅkte
 yaḥ kālaḥ sa truṭiḥ smṛtaḥ
śata-bhāgas tu vedhaḥ syāt
 tais tribhis tu lavaḥ smṛtaḥ
Synonyms: 
trasareṇu-trikam — combination of three hexatoms; bhuṅkte — as they take time to integrate; yaḥ — that which; kālaḥ — duration of time; saḥ — that; truṭiḥ — by the name truṭi; smṛtaḥ — is called; śata-bhāgaḥ — one hundred truṭis; tu — but; vedhaḥ — called a vedha; syāt — it so happens; taiḥ — by them; tribhiḥ — three times; tu — but; lavaḥ — lava; smṛtaḥ — so called.
Translation: 
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.
Purport: 

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.