Since carbon-14 has a half life of 5,730 years, carbon dating is valid (with increasing plus or minus error ) up to about 60,000 years depending on the carbon content of the sample. Advances in mass-spect equipment has made it more accurate particularly with small samples. Have you been involved in any branch of radiometric dating?
The issue is the results spin off around 1400bc from historical dates. There are those that point out that hc is the real constant if this us true is might explain what is going on with radiometric dating which assumes c is constant. The issue is not the measurement but the interpretation of the measurement taken. If c and h are changing then so is the decay rate.