[RESOLVED] What to implement in this case

[stlaural]

Hi guys, I'm kinda having a brain freeze here thinking about this.
Simple best practice question here I would say.

I have a class to create lets call it "Contract" and it has a type property that can be "Service" or "Product"

If I instantiate a Contract of Service type, it should have a start date and and end date. These two dates should be mandatory but only if the instantiate Contract is a Service.

What are the different ways to implement that ? I thought about using the Factory design pattern so if you ask the factory of a Service type Contract you need to provide the two dates but I kinda find it overkill for just two different cases (Service and Product) Any Ideas of other efficient ways of doing that? I Just can't believe there wouldn't be something else.

Thanks for any suggestion.