It is said that every morning we are born again which is a rebirth from a 'time' point of view. In this regard, Krishna says that Brahma (creator) too has a day and night but of a different time scale (8.17). At the dawn of the day, all manifested (creation) appears and dissolves back at the dusk of the night of Brahma (8.18). This is invariably a cyclic process (8.19). Anything under the influence of time is cyclic.
Krishna says that there exists higher than the un-manifested, yet another un-manifested, eternal, the absolute, which remains untouched by the cycle of cosmic dissolution (8.20). A seed is the best example to understand this verse. Inside a seed an un-manifested tree is hidden and the cycle of seed-tree-seed continues. But, there is a creative force beyond both of them which makes this cycle possible and Krishna is hinting at this creative force that is beyond time.
Understanding something beyond time is a difficult task for the mind. Scientifically, the theory of relativity comes closest to helping us understand this intricate issue. It postulates that if we can ride on a photon travelling at the speed of light, then time stops. A photon that started its journey (about a million years back as per our clock on earth) would feel that time hasn't started ticking for it and the photon's reality doesn't have any element of time.
Krishna says that the supreme being in whom all beings dwell, by whom all this is pervaded is indeed attained by unswerving devotion (8.22) and this is the ultimate goal which is my supreme Abode with no return (8.21). No return (𝙢𝙤𝙠𝙨𝙝𝙖) indicates that one is out of the cycle of time (birth-rebirth). This is attained through devotion to the supreme being which is unconditional love to all.