You need a reference to your HUD Timer to be able to cast to it. A reference can be stored when you spawn an actor, create a widget, which also eliminates the need for a cast. Like Exi mentioned, if the HUD Timer is the HUD Class, you can use the Get HUD function return value as your object. A vague example of what a cast will do is if you have an actor reference to your blueprint, you can use that as your object, and it will give you the blueprint reference, meaning that you can access data specific to that blueprint.
He is not flaming you, and I am with eXi on the fact that you need to study OO programming concepts, blueprints are essentially an OO language, which is why those who have a background in languages like C++ are comfortable right away using blueprints.