The use of naturally occurring radioactive decay of uranium to lead is commonly used to determine the timing of events in the geological past. Improvements in methods mean that we can make these measurements at very high precision and we see some unexplained discrepancies. In order to evaluate one possible discrepancy, we have accurately measured the numbers of each lead isotope in a standard sample of lead wire. We have measured a different relative amount of each lead isotope than was determined previously, and when we use this new value to recalibrate geological dates, we find that some of the persistent discrepancies are resolved.