Output of an acceleration spectrum is computed in a Response Spectrum analysis.
Output of a time history is computed in a Transient Structural analysis.
Tell me about the vibration input data that will apply a force to the structure. Where is the force applied? What is the magnitude of the force? What is the frequency of the vibration? Is the vibration continuously present, like an unbalanced motor, or is the vibration caused by a specific event that starts and ends? Do you have a time history for the input force or do you have an acceleration spectrum?
Best regards Peter