Manava Dharmashastra 1.71

1.71

yadetaditi laukikī vāco yuktiḥ | samudāyena prakrānto'rthaḥ parāmṛśyate | yadetaccaturyugaṃ parisaṃkhyātaṃ catvāri sahasrāṇītyādinā niścitasaṃkhyamādau prāgasmācchalokāt-etasya caturyugasya dvādaśabhiḥ sahasrairdevānāṃ yugamucyate | dvādaśacaturyugasahasrāṇi 'devayugaṃ' nāma kāla ityarthaḥ | sahasraśabdātsvārthe'ṇ | dvādaśa sahasrāṇi parimāṇe yasminniti vigrahaḥ |

This period of the four time-cycles that have been just computed,—twelve thousand such periods are called the ‘time-cycle of the gods.’—(71)

Medhātithi’s commentary (manubhāṣya):
‘Yadetat’ (in the sense of ‘this’) is an ordinary expression; and as a whole it means that which has been spoken of.—‘This period of four Time-cycles that has been computed’—i.e., the exact number of years in which have been definitely determined before this verse,—e.g. in verse 69, where the number of years in the Time-cycles is declared to be four thousand etc., etc.,—‘Twelve thousand’ of this period of four Time-cycles are called the ‘Time-cycle of the Gods’. That is to say the Divine Cycle consists of twelve thousand ‘Four-Cycles.’

The word ‘Sāhasram’ is derived from the word ‘sahasra’ with the reflexive affix ‘aṇ’; and the compound ‘dvādaśasāhasram’ is to be expounded as ‘dvādaśa-sahasrāṇi parimāṇe yasmin’, ‘that in which the measure is that of twelve-thousands’.—(71)