Over the last 66 years numerous determinations have been made of the 187Re half-life and decay constant. With the measurement technology having improved, the determinations over the last 20 years have resulted in close agreement between the four determination methods—direct physical counting and in-growth experiments, and radioisotope age comparisons using molybdenites and groups of meteorites. Thus the 187Re half-life and decay constant values of 41.577 ± 0.12 Byr and 1.6668 ± 0.0034 × 10-11 per year respectively have now been adopted for standard use by the uniformitarian geological community. These values are based on determinations recalibrating Re-Os model ages of molybdenites by forcing them (essentially by circular reasoning) to agree with the U-Pb concordia-Pb-Pb intercept ages of zircons from the same 11 magmatic-hydrothermal systems dating from ca. 90 Ma to ca. 2670 Ma. This value is essentially identical to the best of the recent determinations using groups of iron meteorites. It is also within the ± uncertainty range of the half-life values obtained by the best of the physical direct counting and in-growth experiments. Yet, in spite of such experiments directly measuring 187Re decay and its decay product 187Os respectively, preference has been given to the half-life value determined by forcing the Re-Os data to agree with U-Pb dates. But many unprovable assumptions are also involved, not least being that the radioisotope systems closed at the same time and subsequently remained closed. Furthermore, even this "gold standard" has unresolved uncertainties due to the U decay constants being imprecisely known, and to measured variations of the 238U/235U ratio in terrestrial rocks and minerals and in meteorites. Both of these factors are so critical to the U-Pb method, as well as the additional factor of knowing the initial concentrations of the daughter and index isotopes, so it should not be used as a standard to determine other decay constants. In any case, the half-life of 187Re has been shown to be dependent on environmental conditions such as the degree of ionization. Fluctuations in nuclear decay rates have also been observed to correlate with solar activity for a few other isotopes, while there is evidence decay rates of the radioisotopes used for rock dating have not been constant in the past. This only serves to highlight that if the Re-Os dating method has been calibrated against the U-Pb "gold standard" with all these uncertainties, then it cannot be absolute, and therefore it cannot be used to reject the young-earth creationist timescale. Indeed, current radioisotope dating methodologies are at best hypotheses based on extrapolating current measurements and observations back into an assumed deep time history for the cosmos.