The calculation of time in the scriptures is as follows –

The activity takes place in the ‘atom’, the sixth part of the trident, which is finely visible in the rays of sunlight and moonlight entering the house, the time of that activity is called a ‘moment’ (‘kṣaṇa’). After that process occurs, time gets separated from its place. That divisible time is called the ‘second moment’ and the time between those two moments is called ‘ekalava’. Thus the three Lavas union is called a “nimeṣa” ’ or blinking of an eye. Such three constructions is called a ‘room'(‘kakṣa’). The three ‘kakṣās’ are called a ‘kāṣṭhā’. Thirty kāṣṭhās are called a ‘kaṇā’, thirty Kanas are called a ‘muhūrta’, The time of thirty muhūrta is called a human day (‘mānuṣa divasa’) or ‘Solar  Year’/’saurya varṣa’ (The solar year is of 24 hours and the human day is also of 24 hours. The period of 15 such human days is called a ‘pakṣa ’. A season (‘ṛtu’) of 2 months, an ‘ayana’ of 3 seasons, a month of 2 ‘pakṣas ’, the human year (‘mānuṣa varṣa‟) of an ‘ayana’ is one and the solar year (saurya varṣa) is 360.) Such a period of 1000 human years is called ‘pāda’ (Paad or a phase of 3 lakh 60 thousand solar years). Such four phases are called an ‘yuga’. The names of these 4 phases are as follows –

1. The Process phase (prakriyā pāda) 2. The Auxiliary phase (anuṣaṃga pāda) 3. The Prelude phase (upodghāta pāda) 4. The Destruction Phase (saṃhāra pāda). In that, Satyayuga lasts for a period of one phase (pāda), the era of Satyuga has been described in Vāyu Purāṇa as follows –

catvāryāhuḥ sahastrāṇi
varṣāṇāṃ ca kṛtaṃ yugam ।
tasya tāvacchatī sandhyā
sandhyāṃśaśca tathā vidhaḥ ॥

‘Satyuga‟ lasts for 4,000 human years (mānuṣa varṣa). Its ‘sandhyā‟ (sandhi) is of 400 years and its ‘sandhyāṃśa‟ is of 400 years. In this way, one ‘Satyuga‟ lasted for 4,800 years, one of its quarters i.e. its fourth part is of 1,000 yrs. The age of Satyuga is 1,200 years.

tretā trīṇi sahastrāṇi, saṃkhyayā munibhiḥ saha ।
tasyāpi triśatī sandhyā sandhyāṃ śastriśataḥ smṛtaḥ ॥

‘tretāyuga‟ (Tretayuga) is said to last for 3,000 human years. And including 300 years of sandhyā and 300 years of sandhyāṃśa, the age of tretāyuga is 3,600 human years (mānuṣa varṣa). This tretāyuga experiences two parts, one the process phase (prakriyā pāda) and the other auxiliary phase (anuṣaṃga pāda), Including the years of those two Paadas (pāda) and accordingly the years of sandhyā and sandhyāṃśa, tretāyuga experiences a lifespan of 2,400 years.

dvāpare he sahasre tu varṣāṇāṃ saṃprakīrtitam ।
tasyāpi dviśatī saṃdhyā sandhyāṃśo dviśatastathā ॥
upodaghātastṛtīyastu dvāpare pāda ucyate ।

dvāparayuga (Dvaparyuga) is said to be of 2,000 years, 200 years of sandhi and 200 years of sandhyāṃśa, all these together are said to be 2,400 years. However, this dvāparayuga experiences three phases of time. One is Process Phase (prakriyā pāda), second is auxiliary phase (anuṣaṃga pāda) and third is Prelude phase (upodghāta pāda). The years of these three Paadas and accordingly 200-200 years of sandhyā and sandhyāṃśa are added together and making the age of dvāparayuga is 3,600 years.

kali varṣa sahasrantu prāhuḥ
saṃkhyāvido janā: ।
tasyāpi śatikā saṃdhyā
sandhyāṃśaḥ śatameva ca ।
saṃhāra pādaḥ saṃkhyāścaturtho
vai: kalau yuge ।

Kaliyuga is of thousand years and its sandhi is of 100 years and sandhyāṃśa is of 100 years, thus there are a thousand years, but Kaliyuga experiences four phases (Paadas) of time, one process phase (prakriyā pāda), second auxiliary phase (anuṣaṃga pāda), third prelude phase (upodghāta pāda) and fourth destruction phase (saṃhāra pāda), in this way, Kaliyuga experiences a lifespan of 4,800 years by adding 4,000 years of four Paadas and accordingly 200, 200, 200, 200 years of sandhyā.

sa sandhyāni sahasrāṇi
saṃgarāṣṭra śakti
catvāri tu yugāni ।
etada dvādaśa sāhasraṃ
caturyugaṃ samīritam ।

A quartet of 12,000 human years (mānuṣa varṣa)– including sandhyā and sandhyāṃśa and Yuga is called caturyuga (Chaturyuga).

Number data-

Original Scripture – 4800     3600

Satyuga tretāyuga

pāda — 1 pāda, 2 pāda
Thus
Human year (‘mānuṣa varṣa’) – 1200, 2400
Solar year (‘saurya varṣa’) – 432000, 864000,

2400 1200 years
dvāparayuga Kaliyuga
3 Paada 4 Paada
3600 4800 ………. Thus,
1728000 3456000 Human Days (‘mānuṣa divasa’)

In Chapter 57 of Vāyu Purāṇa, it has been said that the saṃvatsara like mānuṣa varṣa, Saurya, kaucaṃ, saptarṣi, divya etc. even after having an era (Yuga) of four phases (pāda), the age of the era (Yuga) is increasing according to the phase (pāda).

In manusmṛti (Manusmriti), vāyu purāṇa (Vayu Purana), bhāgavata (Bhagwat), mahābhārata (Mahabharata) and kalipurāṇa (Kalipurana), that magnitude has been called yugāṃta (end of an era) and the establishment of the yugāṃtara (next era coming after the present era) has been described. In the second chapter of twelfth canto (skandha) of bhāgavata, it is said that ‘Kaliyuga has started when saptarṣi (Saptarishi) came in maghā nakṣatra (Maghaa Nakshatra or constellation)’, saptarṣi stay in one constellation for a hundred years, according to that, 1800 years of 18 Nakshatras (constellations) from maghā to revatī nakṣatra, then 2700 years of 27 Nakshatras from aśvinī to revatī, then 300 years of 3 nakṣatras from aśvinī to kṛttikā, thus 1800 + 2700 + 300 = 4800 years, Kaliyuga ended after 4000 years, after that 400 years passed after its conclusion (sandhi). after that four hundred years passed since its sandhyāṃśa, in all the almanacs it is written that 5040 years of Kali (yuga) have passed, if counted from the beginning of saptarṣi – maghā yoga, then it will be
240 years after the end of kaliyuga and if counted from the end of the hundred (100) years of saptarṣi – maghā yoga, then it will be 140 years after kaliyuga, the years after this are considered as construction of the house of satyuga (satyuga gṛha nirmāṇa) (garbhastha satyuga), during the years of construction of that house, all the evils of kaliyuga will be eradicated and preparations for the means of satyuga are being made as much as necessary, The formation (birth) of satyuga in the form of house construction (garbhastha) starts in saptarṣi – rohiṇī nakshatra, then it is said in bhāgavata, in vāyu purāṇa, Chapter 66 Verse 413,

„yadā candraśca sūryaśca tathā tiṣya bṛhaspati | eka rātre bhaviṣyanti tathā kṛtyugaṃ bhavet ||413||‟
And in bhāgavata 122/24——
“yadā candraśca sūryaśca tathā tiṣya bṛhaspati eka rāśī sameṣyaṃti tadā bhavati tat kṛtam |122 | 24 |”

It has been said that – when Moon, Sun, puṣya and Jupiter will come in one phase in one night i.e. in one zodiac sign Cancer, then garbhastha satyuga a will be born, according to all the calendars of Bharat (India), such a yoga (or favorable juncture) occurs from 12.44 pm on śrāvaṇa amāvasyā of saṃvat 2000, At that time, before the birth of satyuga, the appearance of bhagavāna kalki, who destroys all unrighteousness and purifies the environment, takes place, this is proved in the scriptures. The existence of bhagavāna kalki as the originator of the great sect religion is believed in the last 200 years in the respective dependent sections, kalki Is considered as Ramakrishna Paramahamsa in Bengal, śrī jī śrī prāṇanātha jī of praṇāmī sect in Assam and Sindh province, In Kathiawar of Gujarat, śrī svāmī nārāyaṇa bhagavān, who purified the people by following the religions like aṣṭāṃga, celibacy, non-violence, truth etc., has been shown the form of kalki. But the divine actions of bhagavāna kalki, riding the divine horse, killing the demons with divine weapons by the army, changing the time and atmosphere of the earth, etc., all these are mentioned in many great scriptures, kalki fully bearing these symbols is mentioned in the name of Vishnuyash in Sambhal village, ‘will appear from a Brahmin woman Sumati as soon as half of the year of saṃvat 1999 is over’, this is the hope that the society of present day global scholars is keeping, and this is true, almost 50 percent of the people living on earth, be it terrestrial, forest, aquatic, wind, etc. will be destroyed due to the great war, great diseases and divine outbreaks. In which the religious Golden Age (satyuga) living beings will survive, the rest will be consumed in the destruction.

In vāyu purāṇa and bhāgavata etc., the signs of the end of Kaliyuga and the beginning of Satyuga are definitely visible at present.
The signs of the end of Kali (Yuga) are in vāyu purāṇa Chapter 56, Verses 31 to 125—both physical and spiritual signs are given—

Violence, asceticism, untruth, illusion, killing of an ascetic, Maraki’s disease, death from hunger, absence of rain, destruction of countries, death in the womb, death of youth, death of  middle-aged people, people being irreligious, immoral, less bright, untruth. Will start speaking, Brahmins will be those who wish evil, will be uneducated and misdeeds, people will be afraid of being guilty of their deeds, living beings will be afraid of jealousy, anger, unforgivable greed, fraud etc. The minds of human beings will always remain restless, Brahmins will not read the Vedas, Dwijatis will not perform yajña (Yagya ), kṣatriya (Kshatriyas) and vaiśya (Vaishyas) will deviate from their duties. śudra and antyaja will have sexual intercourse with Brahmins, they will sleep, eat etc. with cāṃḍāla (Chandalas). All the kings will be of petty nature and will become śudra (Shudras), will be promoters of hypocrisy, the subjects will become child-killers and abortionists, age, intelligence, forced nobility will be destroyed, śudra will do the deeds of Brahmins (will become virtuous). Thieves will follow the orders
of the king, earthly people will become thieves, servants will become destroyers of friends, women will become devoid of modesty, without penance and vows, without husbandry, women who consume alcohol, meat and spread illusion, there will be an increase in scoundrels, cows will be destroyed, sages will not be seen anywhere.