Not to be Captain Obvious or anything, but Shakespeare's Sonnet 73 emphasizes that time is divided into the past, the present, and the future. We know—we just blew your minds. But seriously, just check out how carefully each of the poem's three quatrains make sure to include each of these three periods of time. All three begin in the present, but then portray this present as a decayed version of a healthier past. Each quatrain also gives hints about the very different future that is coming. But what about the couplet? If you take a close look, you'll notice that the past is missing. What's up with that, Big Willy?
The couplet doesn't contain any reference to the past because it's all about accepting what you've lost and what you've got left, and—all right, already—getting on with life.
The couplet doesn't contain any reference to the past because the speaker is in denial about how much he has lost.