422
Śrīmad-Bhāgavatam
[Canto 3, Ch. 11]
PURPORT
The years of the demigods are equal to 360 years of humankind. As will be clarified in the subsequent verses, 12,000 of the demigods' years, including the transitional periods which are called  yuga-sandhyās , comprise the total of the aforementioned four millenniums. Thus the aggregate of the above-mentioned four millenniums is 4,320,000 years.
TEXT 19
चत्वारि त्रीणि द्वे चैकं कृतादिषु यथाक्रमम् । संख्यातानि सहस्राणि द्विगुणानि शतानि च ॥१९॥
catvāri trīṇi dve caikaṁ kṛtādiṣu yathākramam saṅkhyātāni sahasrāṇi dviguṇāni śatāni ca
catvāri—four; trīṇi—three; dve—two; ca—also; ekam—one; kṛta-ādiṣu—in the Satya-yuga millennium; yathā-kramam—and subsequently others; saṅkhyātāni—numbering; sahasrāṇi—thousands; dviguṇāni—twice; śatāni—hundreds; ca—also.
TRANSLATION
The duration of the Satya-yuga millennium equals 4,800 years of the demigods; the duration of the Tretā-yuga millennium equals 3,600 years of the demigods; the duration of the Dvāpara-yuga millennium equals 2,400 years; and that of the Kali-yuga millennium is 1,200 years of the demigods.
PURPORT
As aforementioned, one year of the demigods is equal to 360 years of the human beings. The duration of the Satya-yuga is therefore 4,800 x 360 or 1,728,000 years. The duration of the Tretā-yuga millennium is 3,600 x 360 or 1,296,000 years. The duration of the Dvāpara-yuga millennium is 2,400 x 360 or 864,000 years. And the last, the Kali millennium, is 1,200 x 360 or 432,000 years.