1.34. Optionally, mental equanimity may be gained by the even expulsion and retention of energy.
Yogic time runs in cycles. The largest cycles are called Yugas, which chart the rise and fall of human civilization. Depending on your interpretation, this can last as long as the precession of the equinox, or up to trillions of years on the cosmic scale. The Vedic Astrologer, Swami Sri Yukteswar charted the cycles by equinox procession, so I tend use that, given my astrology background.
On a personal level, if we are to accept the cycle of death and rebirth according to karmic law, we get the next form, reincarnation.
OK, now assuming that there are some of you that take that as complete crap, let me break it down on a secular level. From the standpoint of astronomy, we do not know for sure that the universe itself runs in cycles of the big bang and the big crunch and so on. From my understanding, we know that stars revolve around super-massive black holes at the center of the galaxy. We have the obvious changing of the seasons as the earth revolves around the sun. Day becomes night, night becomes day.
Biologically speaking, our body keeps track of time via the circadian cycle of wake and sleep. Finally, we get to the smallest cycle in Yoga, and therefore the most important: breath.
This is why Yoga puts so much emphasis on breath. If we can control our breath through the smallest cycle, it gives us control of larger cycles, and therefore we can change our karma over time.
Prana is the universal energy, analogous to Chi in Taoism, that gives animation and volition to life. The cycle of breath takes in this energy and feeds the body. Have you every seen someone break five cinder blocks just by using their forehead? No you haven't, because they are probably dead or in the hospital with a concussion. With the use of the practice of pranayama, or breath control, the Yogi can achieve balance though breath techniques.