Dear Cillus,
At this moment it is not possible to pass a primitive variable (strings, numbers, dates) to a microflow or nanoflow when calling from a custom or pluggable widgets. Only object can be passed.
The way to solve the problem, it to use a context object with the desired (date) attributes. Use the context object in a data view and place your widget inside. When calling the flow you will automatically receive the object as an input parameter.
Cheers, Andries