  The notions of time and causality are revisited, as well as the A- and
B-theory of time, in order to determine which theory of time is most compatible
with relativistic spacetimes. By considering orientable spacetimes and defining
a time-orientation, we formalize the concepts of a time-series in relativistic
spacetimes; A-theory and B-theory are given mathematical descriptions within
the formalism of General Relativity. As a result, in time-orientable
spacetimes, the notions of events being in the future and in the past, which
are notions of A-theory, are found to be more fundamental than the notions of
events being earlier than or later than other events, which are notions of
B-theory. Furthermore, we find that B-theory notions are incompatible with some
structures encountered in globally hyperbolic spacetimes, namely past and
future inextendible curves. Hence, GR is favorable to A-theory and the notions
of past, present and future.
