Time is measured in astronomy
; it's measured in the military
by the decay
ing bits of heavy atoms; in the wristwatch, by the throbbing of a tiny sliver of quartz
... and in your head, it's measured by the differences between each indefinable instant and the next; by the unsteady syncopation
of every orderly, namable thing as it passes through consciousness into experience.
Like all "underlying," "atomic" aspects of the universe, time is, at it's core, a product of the particular mechanics of human consciousness, specifically (I believe) due to the properties of propagation delay in feedback loops constituted by nerve cells in the brain.
It's common to perceive that time "speeds up" as you get older, that time flies when you are having fun, that a watched pot never boils. These are natural consequences of the process of abbreviation which is evident in human learning.
Just as we learn languages by learning and internalizing larger and larger combinations of sound-producing muscle movements (accents, by the way, are the products of accidental misuse of the learned sound combinations of our native language when speaking a new one), we learn life by abbreviating it as well. When we are young, time is huge. As we grow older, we learn. Repetition collapses minutes into hours - dressing, meals, travel, classes, homework, commuting, seasons, all of them eventually become comparably identical and are only understood or thought of in terms of themselves. In effect, the regular features of our lives disappear in the same way that features disappear from anything that you stare at while keeping your eyes absolutely still. Our minds, after all, are concerned primarily with problems - the solutions to which become the habits, large and small, which become our personalities and constitute the larger portion of our lives. Having little to intervene in the successful execution of our routines leaves us in a state in many ways comparable to sleep, which is, timeless.
It is furthermore not uncommon to find that "in one sense" time has virtually flown, and yet "at the same time" feel intervening weeks/months/years in arduous detail. This conceptual double exposure is a natural effect of the similar multiplicity of distinct events we use to understand how time is passing in our lives. It is no surprise that in moments of extreme crisis, time seems to slow down extremely, even to the point of stopping. "Normal" life goes in cycles, small, large, and tremendous, and everything in between.
Then there are those utterly visceral moments of disaster that haunt the lives of the unfortunate, when every cycle is broken; when anything might happen. In these, before a second goes, a dilated eternity passes.
Before the invention of the clock, time itself was actually very different. Aside from the unavoidable cycles of the day and the seasons, time was remarkably ungrounded. The firm notion of time being linear is a comparatively recent invention. One phrase from an anthropology class sticks with me, about aboriginal cultures - measuring time in circles, or in cycles...
The U.S. Naval Observatory ("The Official Source of Time for the Department of Defense and the Standard of Time for the United States") has been maintaining a wonderful website for years and years now (http://tycho.usno.navy.mil/). I quote:
"The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination. Modern electronic systems, such as electronic navigation or communications systems, depend increasingly on precise time and time interval (PTTI). Examples would be the ground-based LORAN-C navigation system and the satellite-based Global Positioning System (GPS). These systems are based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of 10 feet. In fast communications, time synchronization is equally important. All of these official systems are referenced to the USNO Master Clock."
"These clocks are distributed over 20 environmentally controlled clock vaults, to ensure their stability. By automatic intercomparison of all clocks every 100 seconds, the USNO time scale can be computed which is not only reliable but also extremely stable. Its rate does not change by more than about 100 picoseconds (0.000 000 000 1 seconds) per day from day to day.
On the basis of this computed time a clock reference system can be steered to produce clock signals which serve as the USNO Master Clock. The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen masers are extremely stable clocks over short time periods (less than one week). They provide the stability, reliability and accessibility needed to maintain the accuracy of the Master Clock system."
There's also a wonderful essay entitled "How do we find the very best clock?" Almost like a hint, this is a more subtle a problem than you might guess.