Abstract: Zero-time of nuclear events can be determined by activity ratio of fission products. The calculation method of zero-time of nuclear events was investigated by measuring ⁹⁵Zr-⁹⁵Nb activity ratio. The study results show that the zero-time can be determined accurately by ⁹⁵Zr-⁹⁵Nb activity ratio. In addition, the influence of the ⁹⁵Nb^m decay branch can be ignored. The deviations of zero-time between the result determined by ⁹⁵Zr-⁹⁵Nb activity ratio and reference value are less than 1 d and 10 d for decay of 100 d and 1 a from zero-time respectively. The uncertainty of zero-time increases rapidly as the increasing of interval time between measurement start time and zero time. The uncertainty interval is asymmetric and the positive uncertainty is larger than negative uncertainty. The positive uncertainty approaches infinity when the ⁹⁵Zr-⁹⁵Nb equilibrium is reached.