  As the quality and quantity of astrophysical data continue to improve, the
precision with which certain astrophysical events can be timed becomes limited
not by the data themselves, but by the manner, standard, and uniformity with
which time itself is referenced. While some areas of astronomy (most notably
pulsar studies) have required absolute time stamps with precisions of
considerably better than 1 minute for many decades, recently new areas have
crossed into this regime. In particular, in the exoplanet community, we have
found that the (typically unspecified) time standards adopted by various groups
can differ by as much as a minute. Left uncorrected, this ambiguity may be
mistaken for transit timing variations and bias eccentricity measurements. We
argue that, since the commonly-used Julian Date, as well as its heliocentric
and barycentric counterparts, can be specified in several time standards, it is
imperative that their time standards always be reported when accuracies of 1
minute are required. We summarize the rationale behind our recommendation to
quote the site arrival time, in addition to using BJD_TDB, the Barycentric
Julian Date in the Barycentric Dynamical Time standard for any astrophysical
event. The BJD_TDB is the most practical absolute time stamp for
extra-terrestrial phenomena, and is ultimately limited by the properties of the
target system. We compile a general summary of factors that must be considered
in order to achieve timing precisions ranging from 15 minutes to 1 microsecond.
Finally, we provide software tools that, in principal, allow one to calculate
BJD_TDB to a precision of 1 microsecond for any target from anywhere on Earth
or from any spacecraft.
