Units of time is required to measure a particular time interval. And according to the usage, either in general or some scientific uses, units of time are selected. In Vedas, time is called Kala. Vedic astronomy also give a division of time upto lowest division of time. Now, let's discuss the time calculation and divisions(unit) defined in Vedas:
Parmanu(Atom) = 16.8 μs
Anu(2 Parmanu) = 33.7 μs
Trasarenu(3 Anu) = 101 μs
Truti(3Trasarenu) = 304 μs
Vedha(100 Truti) = 30 ms
Lava(3 Vedha) = 91 ms
Nimesha(3 Lava) = 0.273 s
Ksanas(3 Nimesa) = 0.82 s
Kasthas(5 Ksana) = 4.1 s
Laghu(15 Kastha) = 1 minutes
Nadika(15 Laghu) = 15.3 minutes
Muhurta(2 Nadika) = 30.7 minutes
Yama = 7.5 Muhurta
Daylight = 4 Yamas/Praharas = Night
8 Yamas/Praharas = 1 Day = 1 Daylight + 1 Night
Paksha(1 Fortnight) = 15 Days
2 Paksha = 1 Month(Maasa)
6 Maasa = 1 Ayanam
2 Ayanam = 1 Year
After these, next level of measurement is Yuga, then Chaturyuga, then Manavantaras, then Kalpa, then Brahma year. All these are used to determine the age of the universe.
In Bhagwat Puran also, these units are discussed and defined. Let's discusses those verses:
Verse 3.11.6
त्रसरेणुत्रिकं भुङ्क्ते यः कालः त्रुटिः स्मृतःI शतभागस्तु वेधः स्यात्तैस्त्रिभिस्तु लवः स्मृतःII
Measure of time which travels across a composite of three Trasarenus is known as Truti (Truti = 3 Trasarenus); a Vedha consistes of 100 Trutis (Vedha = 100 Truti); while three Vedhas constitute what is known as a Lava (Lava = 3 Vedhas).
Verse 3.11.7
निमेषस्त्रिलवो ज्ञेय आम्नातस्ते त्रयः क्षणःI क्षणान् पञ्च विदुः काष्टां लघु ता दश पञ्च चII
A composite of three Lavas is known as Nimesa (Nimesa = 3 Lavas); while three Nimesa compose together to form Ksana (Ksana = 3 Nimesa) and a composite of five Ksanas form Kasthas and fifteen Kasthas go to make Laghu (Laghu = 15 Kasthas).
Verse 3.11.8
लघूनि वै समाम्नाता दश पञ्च च नाड़िकाI ते द्वे मुहूर्तः प्रहरः षड्यामः सप्त वा नृणाम्II
Fifteen Laghus taken together are called Nadika (Nadika = 15 Laghus) and couple of Nadikas constitue of one Muhurata (Muhurata = 2 Nadikas) while six or seven Nadikas make a Prahara, Prahara which forms one quarter of day or night of human beings i.e. 1Day(light)= 4 Prahara = 1 Night.
In Bhagwat Puran also, these units are discussed and defined. Let's discusses those verses:
Verse 3.11.6
त्रसरेणुत्रिकं भुङ्क्ते यः कालः त्रुटिः स्मृतःI शतभागस्तु वेधः स्यात्तैस्त्रिभिस्तु लवः स्मृतःII
Measure of time which travels across a composite of three Trasarenus is known as Truti (Truti = 3 Trasarenus); a Vedha consistes of 100 Trutis (Vedha = 100 Truti); while three Vedhas constitute what is known as a Lava (Lava = 3 Vedhas).
Verse 3.11.7
निमेषस्त्रिलवो ज्ञेय आम्नातस्ते त्रयः क्षणःI क्षणान् पञ्च विदुः काष्टां लघु ता दश पञ्च चII
A composite of three Lavas is known as Nimesa (Nimesa = 3 Lavas); while three Nimesa compose together to form Ksana (Ksana = 3 Nimesa) and a composite of five Ksanas form Kasthas and fifteen Kasthas go to make Laghu (Laghu = 15 Kasthas).
Verse 3.11.8
लघूनि वै समाम्नाता दश पञ्च च नाड़िकाI ते द्वे मुहूर्तः प्रहरः षड्यामः सप्त वा नृणाम्II
Fifteen Laghus taken together are called Nadika (Nadika = 15 Laghus) and couple of Nadikas constitue of one Muhurata (Muhurata = 2 Nadikas) while six or seven Nadikas make a Prahara, Prahara which forms one quarter of day or night of human beings i.e. 1Day(light)= 4 Prahara = 1 Night.