This paper presents a time-dependent concurrent reliability-based design optimization (TDC-RBDO) method integrating the time-variant B-distance index to improve the confidence level of design results with a small amount of experimental data. The time-variant B-distance index is first constructed using the extreme values of responses. The Hist Loop CDF (HLCDF) algorithm is then presented to calculate the time-variant B-distance index with high computational efficiency. The TDC-RBDO framework is provided by integrating the time-variant B-distance index and time-dependent reliability. The extreme value moment method (EVMM) is implemented to speed up the procedure of the TDC-RBDO. The case of a harmonic reducer is employed to elaborate on the proposed method.