I understand that the most accurate clocks in the world are the atomic clocks. Unfortunately, trying to undertand why they are the most accurate is all a little confusing !!! Go to www.howstuffworks.com which explains the ins and outs of the atomic clocks. Good Luck!
American and German scientists have developed the new all-optical atomic clock which can tell time to a femtosecond.
The optical 'shifts' in atoms are faster than the icrowave-frequency 'shifts' in cesium (which has been used to tell time accurately since 1967, using nanoseconds). A femtosecond laser is capable of counting the optical shifts in a single, cooled mercury ion - they happen 1.064 quadrillion times a second, or once a femtosecond.
All may be correct. To go slightly further afield, as I understand it, the most accourate timekeeping device in the known universe is a pulsar star (an ultraheavy collapsed ex-star)
Or I could be wrong. Either way, it's more accurate than me saying "One thousand, two thousand, three thousand...................."