Like many real-world models, this type of situation can be modeled both continuously and discretely. For example, if you defined the problem as a series of equations defining the rates at which items arrived and departed it would be continuous, but if you modeled it at a more microscopic level, showing individual arrivals and departures of trucks, it would be discrete. Then, going further, if you were to model the actual acceleration of the trucks as they departed the loading bay, it would become continuous again.
In general, job shop-type simulations are usually modeled as a series of discrete events. You may want to refer to queuing theory.
Note that there is an open source project called Facsimile that does discrete event simulation and is intended to do the kind of simulation you are aiming at. There are also a lot of commercial systems that specifically do industrial logistics and job shop simulation; for example AnyLogic is one such package.
You can also use generic simulation languages like Modelica to create the simulation.
In general, it is good idea to use simulation software rather than write your own simulation in many cases because there are many subtleties to it that you would not expect. Using an existing simulation package you can create your simulation in a fraction of the time it would take to program it and the result is potentially much more reliable with less risk of bugs.