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Mixed ²³⁵U–²³³U–²⁰⁵Pb(–²⁰²Pb) tracers for U–Pb isotope-dilution isotope ratio mass spectrometry have been prepared under the auspices of the EARTHTIME Initiative. The methods and results for the preparation and calibration of the U/Pb ratio and isotopic abundances are given, and the various sources of uncertainty are discussed and quantified. The accuracy of the EARTHTIME U–Pb tracer isotopic composition can be traced back to SI units via a series of assay and isotopic composition reference materials combined with the experiments described herein. The parameters used in calculating U/Pb ratios (and inferentially U–Pb dates) have correlated uncertainties that result in a total uncertainty contribution to ²⁰⁶Pb/²³⁸U dates of ± < 0.03% (95% confidence). For suitable terrestrial materials such as zircon, when other sources of uncertainty have been minimised (e.g., open-system behaviour, ²³⁸U/²³⁵U …