The understanding of these fluctuations will be promoted by our brief glance at the constant errors in retrospective time-appreciation.
And here it is necessary to distinguish between the sense of duration which we have during any period, and the retrospective sense which survives the period, for these do not necessarily agree.

The former rests mainly on our prospective sense of time, whereas the latter must be altogether retrospective.[121] Our estimate of time as it passes is commonly said to depend on the amount of consciousness which we are giving to the fact of its transition.

Thus, when the mind is unoccupied and suffering from _ennui_, we feel time to move sluggishly.

On the other hand, interesting employment, by diverting the thoughts from time, makes it appear to move at a more rapid pace.

This fact is shown in the common expressions which we employ, such as "to kill time," and the German _Langweile_.
Similarly, it is said that when we are eagerly anticipating an event, as the arrival of a friend, the mere fact of dwelling on the interval makes it appear to swell out.[122] This view is correct in the main, and is seen, indeed, to follow from the great psychological principle that what we attend to exists for us more, has more reality, and so naturally seems greater than what we do not attend to.