Achieving better than one-minute accuracy in the Heliocentric and Barycentric Julian Dates

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Abstract
As the quality and quantity of astrophysical data continues 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 time standards with precisions of considerably better than a 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 the time system always be reported. We summarize the rationale behind our recommendation to use BJD_TDB, the Barycentric Julian Date in the Barycentric Dynamical Time standard, which is the most practical absolute time reference 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.

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