We demonstrate that in the study of time-resolved photoluminescence the signatures of biexcitons in QDs can manifest themselves even when the system is excited by long laser pulses with their duration of the order of several tens of picoseconds. To describe the results of our measurements we propose an elegant theoretical model. It is based on the introduction of auxiliary sources in the rate equations. The parameters of the sources are determined by fitting the analytical solution of these equations that describes the full emission of the system to the experimental results obtained. Having found the parameters of the auxiliary sources we can determine the temporal dependences of the radiation from excitons and biexcitons separately. We find the spectral positions of the exciton and biexciton emission bands, their widths, and the biexciton binding energy.