How to define time condition in frequency analysis

Hello; I have generated an acoustic_solid interaction model in frequency domain to measure the acoutic velocity of the formation surrounding a borehole. A monopole point source is located inisde the acoustic domain. I have used the FFT to transfer the frequency domain data to time domain. my question is that how I can define the condition time bigger than zero (t>0) for my point source in frequency domain? since my assumption is that in intial time (t=0), my point source is not excited inside the borehole. Thanks;