Things are sacred because they persist beyond our measures of time. As Eliade would say, the sacred "resists time; its reality is coupled with perenniality (Eliade 4)." It is the notion that things which are sacred persist, and that they do so in ways unlike our normal reckoning, that draws out the difference between linear and cyclic time, allowing one to be a profane measure, and one to be sacred, respectively.
Cyclic time is a constant return, usually via natural cycles (sun, moon, stars, etc.) to creation and cosmogony. Essentially, the world is "reset" and begins anew each time the cycle repeats itself (Eliade 51, 57-58). Cycles can be narrow (such as dawn to dawn) or broad (lifetimes, zodiological eras), but are independent of linear timelines and remain innately tied to religion. They can be thought of as "transhistorical." (Eliade, 147)
Linear time is an historical, certain measure of time, marked out by beginnings and endings. In linear time, one can always specify a quantity of "when." Also, this is communicative time, exact enough to specify when things take place to a wide variety of people with different backgrounds.
Ritual moves in cyclic time, drawing the participants back to "the beginning" or "the primordial," as each "creation repeats the re-eminent cosmogonic act, the Creation of the world (Eliade 18)." Because ritual always takes place at the center of the world (ibid.), ritual (and sacrifice in particular) can be seen primarily as re-cycling the participants, and by extension the cosmos, through the mythic, sacred cycle. It forces us to begin the cycle again, regardless of where we are in linear time, or how far removed from the primordial act we may truly be.
Linear time and cyclic time can interrelate: a single founding date, when known, can serve as a benchmark and a renewal point as well. Consider wedding anniversaries, marked in cyclical time as points of renewal (where vows may be respoken and reaffirmed, or when large displays of love similar to the love displayed on "Day One" can be expected), but also marked quantitatively by hears, which deepen the meaning of the cycle’s repetition.
The cyclic time of ritual serves to renew the linear time we exist in: it gives the span of our lives meaning, and allows us to participate in things ab origine, at the beginning of time. Likewise, entering the cyclic time of ritual allows us access to something we know we can never truly have: the immortality of the gods (Eliade 36). By entering the time span of the gods, we also enter their world. As cyclic and linear time interact, so do we experience the transcendent reality of perennial deity.