Not sure if this input is useful. In SPICE simulation, you have no control over the timestep. The timestep dynamically adjusts throughout the simulation in SPICE-based simulators. Fixed timestep in general only found in PieceWise-Linear simulator.
Regardless of your actions, you cannot force the simulation to run, for example, exactly at 5ns, 10ns, or 15ns with a fixed step. For discrete simulation, approach is to execute discrete operations at consistent intervals. Generally, to achieve this timing consistency, Qspice involves a temporal timestep (ttol) in Trunc(), which aims to refine the simulation step to a precise enough level before reaching the desired interval time point to perform discrete updates, rather than to force time to fly to next timestep you decided.
In short, SPICE determines the next timestep based on the simulation’s need to converge on a solution. Your option is to make this step smaller but never larger. That’s why you don’t have precise control over the timestep. SPICE doesn’t know what the next timestep should be; it depends on whether convergence can be achieved (in general, it has a target maxstep, if this step can converge, it just take this step). If convergence is possible but you assign a smaller timestep in ttol, it can reduce this step to ttol accordingly. However, if your ttol setting is larger than that, SPICE simply ignores it.