Text 18]
Calculation of Time, from the Atom
421
TRANSLATION
O spiritually powerful one, you can understand the movements of eternal time, which is the controlling form of the Supreme Personality of Godhead. Because you are a self-realized person, you can see everything by the power of mystic vision.
PURPORT
Those who have reached the highest perfectional stage of mystic power and can see everything in the past, present and future are called  tri-kāla-jñas . Similarly, the devotees of the Lord can see clearly everything that is in the revealed scriptures. The devotees of Lord Śrī Kṛṣṇa can very easily understand the science of Kṛṣṇa, as well as the situation of the material and spiritual creations, without difficulty. Devotees do not have to endeavor for any  yoga-siddhi , or perfection in mystic powers. They are competent to understand everything by the grace of the Lord, who is sitting in everyone's heart.
TEXT 18
मैत्रेय उवाच
कृतं त्रेता द्वापरं च कलिश्चेति चतुर्युगम् । दिव्यैर्द्वादशभिर्वर्षैः सावधानं निरूपितम् ॥१८॥
maitreya uvāca kṛtam tretā dvāparam ca kaliś ceti catur-yugam divyair dvādaśabhir varṣaiḥ sāvadhānam nirūpitam
maitreyaḥ uvāca —Maitreya said;  kṛtam —the age of Satya;  tretā —the age of Tretā;  dvāparam —the age of Dvāpara;  ca —also;  kaliḥ —the age of Kali;  ca —and;  iti —thus;  catuḥ-yugam —four millenniums;  divyaiḥ —of the demigods;  dvādaśabhiḥ —twelve;  varṣaiḥ —thousands of years;  sāvadhānam —approximately;  nirūpitam —ascertained.
TRANSLATION
Maitreya said: O Vidura, the four millenniums are called the Satya, Tretā, Dvāpara and Kali yugas. The aggregate number of years of all of these combined is equal to twelve thousand years of the demigods.