There are a few different time computation intervals in SWMM that you would need to consider. Those are: reporting, dry weather, wet weather, and routing. If you are simulating a single event, then the wet weather interval is closes to the time computation interval in HEC-HMS. If you are simulating times between events as well, then you will also need to consider the dry weather interval. The routing time interval is used to introduce stability to the hydraulic component of the model and needs to be much smaller (~1-3 seconds, depending on the Courant condition) for the dynamic wave solver than for the kinematic wave solver.
These can be specified by selecting "Options" in the project panel and double-clicking "Time Steps" at the bottom of the project panel.
I would highly recommend reading the published SWMM manuals and asking questions on the list serve which can both be found at " https://www.openswmm.org/ "