When the WTT is created paths are created for freight alongside the naturally more regular nature of passenger traffic. Freight trains will often operate their services outside of these planned paths if requested and they can be accommodated. Earlier or later.
If you want to simulate a particular service on a particular day have a look at Realtime Trains and see how the train was pathed on a particular day as opposed to trying to use the published WTT times. Research is your friend 🙂