For a long while, "time" has served as the ontological— or rather
ontic— criterion for naively distinguishing the different regions of
beings. "Temporal" beings (natural processes and historical events) are
separated from "atemporal" beings (spatial and numerical relationships).
We are accustomed to distinguishing the "timeless" meaning of propositions
from the "temporal" course of propositional statements. Further, a "gap"
between "temporal" being and "supratemporal" eternal being is found, and
the attempt made to bridge the gap. "Temporal" here means as much as being
"in time," an obscure enough definition to be sure. The fact remains that
time in the sense of "being in time" serves as a criterion for separating
the regions of Being. how time comes to have this distinctive ontological
function,and even withwhat right precisely something like time serves as
such a criterion, and most of all whether in this naive ontological
application of time its geninely possible ontological relevance is
expressed, has neither been asked nor investigated up to now.
"Time," especially on the horizon of the common understanding of it,
has chanced to acquire this "obvious" ontological function "of itself,"
as it were, and has retained it to the present day.

Martin Heidegger (1902-1985),
Being and Time (1927), translated by Joan Stambaugh
[Heidegger, Basic Writings, Ed. David F. Krell (1977), pp. 60-61]