In the sciences generally, time is usually defined by its measurement: it is simply what a clock reads. Physics in particular often requires extreme levels of precision in time measurement, which has led to the requirement that time be considered an infinitely divisible linear continuum, (i.e. composed of discrete and indivisible units). With modern atomic time standards like TAI and UTC (see the section on Time Standards) and ultra-precise atomic clocks (see the section on Clocks), time can now be measured accurate to about 10−15 seconds, which corresponds to about 1 second error in approximately 30 million years. (http://www.exactlywhatistime.com/)
