GLOS­SARY: Drift

No two clocks, no mat­ter how pre­cise, will keep the same time (espe­cially if you are mea­sur­ing time down to the nanosec­ond). Assum­ing two clocks are syn­chro­nized at a point in time, drift is the dif­fer­ence in time between the two clocks after the ini­tial time setting.

In mechan­i­cal clocks, drift is caused by changes in tem­per­a­ture, poor work­man­ship, fric­tion, etc. Drift in mechan­i­cal clocks is addressed typ­i­cally with a com­pen­sat­ing device. For exam­ple, you can cor­rect the amount of drift in a grand­fa­ther clock by adjust­ing the length of the pendulum.

Most elec­tronic watches and inter­nal com­puter clocks use a quartz crys­tal oscil­la­tor. While quartz clocks are sig­nif­i­cantly more accu­rate than mechan­i­cal clocks, they are still sub­ject to drift caused by changes in tem­per­a­ture, fluc­tu­a­tions in the cur­rent, degra­da­tion of the crys­tal, etc. You could com­pen­sate for drift by vary­ing the volt­age or reg­u­lat­ing the tem­per­a­ture, but the cost would be pro­hib­i­tive for most appli­ca­tions. So, the solu­tion for drift, espe­cially for com­put­ers, is to peri­od­i­cally syn­chro­nize and reset the clock with an exter­nal time source.