Concept of Time Division in Ancient India

Time division in Ancient India was measured to the least possible fraction of a second.
The smallest measurable unit of time is Paramanu.
ancient indian time calculation in vedas

Time calculation and divisions (as described in Manu Smriti and Vedas) in Ancient India :

Small units of time used in the Vedas
Paramāṇu (atom)ca. 16.8 µs
Aṇu2 Paramāṇuca. 33.7 µs
Trasareṇu (particles observed in sun light)3 Aṇuca. 101 µs
Truṭi3 Trasareṇuca. 1/3290 s = ca. 304 µs
Vedha100 Truṭica. 30 ms
Lava3 Vedhaca. 91 ms
Nimeṣha (blink of eye)3 Lavaca. 0.273 s
Kṣhaṇas3 Nimeṣa0.82 s
Kāṣṭhās5 Kṣhaṇaca. 4.1 s
Laghu15 Kāṣṭhāca. 1 min
Nāḍika15 Laghuca. 15.3 min
Muhurta2 Daṇḍaca. 30.7 min
Yāma7.5 Muhūrta
day(light)4 praharas or 4 yamas
day8 yamas = 1 day(light) + 1 night

15 such days = 1 Paksha (fortnight)

This Paksha (brigher lunar cycle) is day for Pitrus (departed ancestors) and darker lunar cycle is their night.

2 Paksha = 1 Maasa (month)
6 Maasa = 1 Ayanam
2 Ayanam = 1 Year

Here, Uttarayanam (Between 15 January – 15 July, when sun rises inclined towards north-east) is said to be day for Gods and Dakshinayanam (remaining 6 months when sun rises inclined towards south-east) is said to be their day.
That is how, human year is equivalent to 1 day in Gods.

An epoch or a yuga is the next higher level of measurement. 1200 Deva years constitute the Kaliyuga or the present epoch that we are believed to be living in; 2400 deva years make up the Dwapara yuga that preceded kali yuga; 3600 years made up the Treta yuga and 4800 the Krita yuga respectively.
Thus, the length of the Kaliyuga is 1200*360 i.e. 432,000 human years.
A cycle of 4 yugas is referred to as the Maha Yuga. A cycle of Maha yugas lasts for 12,000 deva years or 12,000* 360 i.e. 4,320,000 human years.

For higher calculations : Refer to Age of Universe and Brahma

Lifetime of one Brahma is known as ‘Para‘ and it is divided into two halves, each called as ‘Parardham‘.

Right now, we’re in dwitiya parardham (2nd half of current brahma’s lifecycle).
He has completed 50 years of age and entered 1st day of his 51st year.
In that 1st day (kalpa), it is Sweta Varaha Kalpa, Vaivasvata Manvantara (7th out of 14), 28th Maha Guya (out of 71), 1st quarter of Kali Yuga, in which 5113 years have passed so far.