sahastrayugaparyantamaharyadbrahmano viduhu |
raatrim yugasahastraantaam tehoraatravido janaahaa || 17 ||

They who know the day of Brahma comprising a thousand yugaas, the night of Brahma comprising a thousand yugaas, those people know day and night.
sahastra : thousand
yuga : yugaas
paryantam : until
ahaha : one day
yat : they who
brahmanaha : of Brahma
viduhu : know
raatrim : one night
yuga : yugaas
sahastraantaam : thousand
te : those
ahoraatravidaha : know day and night
janaahaa : people
We now enter into the topic of cosmic creation with this shloka. Before the topic is taken up, Shri Krishna introduces some words here as a way of introduction. The new words introduced here are sahastra and yuga. Although not mentioned explicitly, we shall also look at the four yugas or ages that are part of the cosmic creation: satya yuga, treta yuga, dvaapara yuga and kali yuga.
The cycle of creation is said to begin with satyayuga or the age of truth. It is followed by the treta yuga and the dvaapara yuga. The last yuga is kali yuga or the age of discord, which is the current yuga. Kali yuga is the shortest yuga comprising 432,000 years. Dvaapara is twice that long, treta is thrice that long, and satya yuga is four times that long. Therefore, the total time of creation is ten times 432,000, which is 4.32 million years. This period of creation is also known as chatur-yuga.
Now, having understood the length of chatur yuga, we can calculate the length of Brahma’s day and night. Shri Krishna says that one day of Lord Brahma equals one thousand yugaas. The interpretation of the word yuga here is traditionally taken to mean a chatur-yuga. So then, a day of Brahma equals one thousand times 4.32 million, which is 4.32 billion years. This is also the length of one night of Lord Brahma.
Let us know dig deeper into the symbolic meaning of this shloka. There are two aspects we need to look at. First, this shloka re-emphasizes that every action we perform always bears fruit, but not necessarily in the current lifetime.
We may try and try very hard, but the result of our action may not show up in this lifetime, it may bear fruit in another lifetime. Alternatively, we may sometimes get into situations that we think we do not deserve. These could be favourable situations (an unexpected financial windfall) or unfavourable situations (a natural disaster). These type of results are due to actions we have performed in a prior lifetime.
The second aspect is to do with the cultivation of dispassion or vairagya towards this world. We perform actions in this world not just for immediate gain, but also to leave behind a legacy, leave behind something that future generations will remember us by. It could be wealth, power, prestige and so on. But in effect, Shri Krishna tells us that nothing will last forever. Everything will eventually end, if not now then certainly in 4.32 billion years. So if this is going to be the case, it is better to cultivate dispassion right now so that we can proceed spiritually.
How exactly does the universe come into being and dissolve? This is explained next.