Eon

Eon-Yug-ਜੁਗ

The following Gurbani verse

ਗੁਪਤੇ ਬੂਝਹੁ ਜੁਗ ਚਤੁਆਰੇ ॥
Guptey boojhoh jug chatuaare || 

clearly states that Eon/Yug/ਜੁਗ is something that needs to be figured out (boojhoh). This means that the earlier conventional belief about what an Eon/Yug/ਜੁਗ is (that is, time period spread across centuries of time), must be false.

The next part of the verse states

ਘਟਿ ਘਟਿ ਵਰਤੈ ਉਦਰ ਮਝਾਰੇ ॥ (ਆਦਿ ਗ੍ਰੰਥ, ੧੦੨੬)
Ghat ghat vartai udar majhare || (Aad Granth, 1026)

meaning thereby that the Eon/Yug/ਜੁਗ is something that manifests inside the Ghat/ਘਟਿ (that is, the abode of the soul). It is precisely for the same reason that another verse that states

ਇਕ ਘੜੀ ਨ ਮਿਲਤੇ ਤਾ ਕਲਿਜੁਗੁ ਹੋਤਾ ॥ (ਆਦਿ ਗ੍ਰੰਥ, ੯੬)
Ik gharhi na milte ta kaljug hota || (Aad Granth, 96)

clarifies that if I am not in union with my inner self (na milte) for even a second (ik gharhi), I am in Kaljug (which is one of the conventional 4 Eons/Yugs). It also means of course, that if one 'is' in unison, then one is not in Kaljug. This goes contrary to the popular belief that the current time period of human race is Kaljug, for everyone.

According to Gurbani, whosoever is under the influence of Kalpana (desires) is in Kaljug, and whosoever has surrendered to the truth (that is, Sat) is in Satjug (another one of the conventional 4 Eons/Yugs). The stage when one is in the womb, and is creating and getting connecting with the physical body is Dwapar (another Eon/Yug). The stage when one leaves this body and proceeds further is called Treta (another Eon/Yug).