Our perception of time during short intervals is affected by inner feelings and psychological changes. Time flies at the beach or ballpark, but the clock seems stuck when we're doing monotonous work or recovering from a painful experience. Is our perception of time over longer periods governed by a similar subjectivity?   Calendar time is thought to represent the experience of moving through life, marking off the days, months, and years at a uniform pace. But this doesn’t square with our own perceptions. In fact, by middle age, most of us sense at the years―which crawled by in our youth are rushing past at ever increasing speeds. Clearly the linear view as it applies to our own experience is misleading. With this in mind, researchers have proposed an alternative, nonlinear method for tracking the apparent velocity of time.   The basic idea of this approach is that an individual judges the length of any given period, especially a longer period such as a month or a year, by comparing it to his or her total life span. A year is 10% of one's total life if one is 10 years old, but a 20-year-old would experience the same calendar year as only half that length of time, a mere 5%. The life percentage of a particular timespan is just as important as actual clock or calendar time.   People tend to overlook the shrinking of years when they are young because of a preoccupation with current concerns and future goals. We become more retrospective at middle age, when settling into more mundane lifestyles, and it is then that a change in the perception of time becomes apparent.   An appreciation of the nonlinear view can help us bridge the gap in perceptual differences between children and adults. When a parent tells a child to “wait a minute” and then keeps them waiting five or ten, the child may experience the wait as being much longer. It makes sense, then, that they grow edgy.