Manava Dharmashastra 1.70

1.70

kṛtayugādanyeṣu tretādiṣu triṣu yugeṣu sandhyāsandhyāṃśasahiteṣu sahasrāṇyekāpāyena vartante hānirapāyaḥ | ekaṃ sahasraṃ hīnaṃ tretāyāṃ kṛtayugāt | evaṃ tretāto dvāparasya-dvāparātkaleḥ | evaṃ ca trīṇi varṣasahasrāṇi tretā, dve dvāparaḥ, ekaṃ kaliriti bhavati | śatāni hīyante sandhyāttadaṃśayoḥ | viśiṣṭo'hnaḥsaṅghāto yugākhyastasya viśeṣāḥ kṛtādayaḥ | tāvacchatīti īkāraḥ smarttavyaḥ | iha smṛtiḥ | tāvatāṃ śatānāṃ samāhāraḥ tāvacchabdasya 'bahugaṇavatuḍatīti' vatvantatvātsaṃkhyāsaṃjñāyāṃ satyāṃ 'saṃkhyāpūrvo dviguriti' dvigusaṃjñāyāṃ satyāṃ-'ṭāpopavādo dvigo' riti ṅīp | tat parimāṇamasya iti | 'yattadetebhya' iti vatup | 'ā sarvanāmna' ityākāraḥ | anyathā bahuvrīhau tāvanti śatāni yasyāḥ, śataśabdasyākārāntatvāt 'ajādyataṣṭāp' iti ṭāpā bhavitavyam | tasminkṛte 'tāvacchatā' iti syādityabhiprāyaḥ |

In each of the other time-cycles, along with their ‘junctures’ and ‘juncture-ends’, the ‘thousands’ and ‘hundreds’ are reduced by one.—(70)

Medhātithi’s commentary (manubhāṣya):
Among the three Time-cycles other then the Kṛta,—i.e. in Tretā and the rest,—along with their ‘junctures’ and ‘juncture-ends’,—the ‘thousands’ are reduced (in each) by one; ‘reduction’ means dimunition. That is in Tretā, the number of ‘thousand,’ is one less than that in Kṛta; similarly the number in Dvāpara is one less than that in Tretā; and that in Kali is one less than that in Thus Tretā consists of three thousand years, Dvāpara of two thousand years, and Kali of one thousand years. Similarly the number of ‘hundred’ goes on diminishing in the ‘junctures’ and ‘juncture-ends’ of the Cycles.

‘Time-cycle’, ‘yuga’, is the name given to a particular aggregate of days; and ‘Kṛtā’ and the rest are particular names of the said ‘Time-cycle’.—(70)