An introduction to mathematics, by A. N. Whitehead.

QUANTITY 247
Our perception of the flow of time and of
the succession of events is a chief example
of the application of these ideas of quantity.
We measure time (as has been said in considering periodicity) by the repetition of
similar events-the burning of successive
inches of a uniform candle, the rotation of
the earth relatively to the fixed stars, the
rotation of the hands of a clock are all examples of such repetitions. Events of these
types take the place of the foot-rule in relation to lengths. It is not necessary to assume
that events of any one of these types are
exactly equal in duration at each recurrence.
What is necessary is that a rule should be
known which will enable us to express the
relative durations of, say, two examples of
some type. For example, we may if we like
suppose that the rate of the earth's rotation
is decreasing, so that each day is longer than
the preceding by some minute fraction of a
second. Such a rule enables us to compare
the length of any day with that of any other
day. But what is essential is that one series
of repetitions, such as successive days, should
be taken as the standard series; and, if the
various events of that series are not taken as
of equal duration, that a rule should be
stated which regulates the duration to be
assigned to each day in terms of the duration of any other day.