The time step is the amount by which the model time advances each time the state variables are updated. A model may have more than one time step.
set.model.step(instance.handle,step.index,step.size)
instance.handle | 
 The handle returned by create.model(), identifying an executable model instance.  | 
step.index | 
 The level of the time step to be set. If a model only has one time step, this is step 1. Higher levels are set to shorter steps.  | 
step.size | 
 The duration for the time step.  | 
None
Jasper Taylor