Forecasting an upcoming release should include “what if we start next week versus today and assume X velocity?”

Seems odd to me for “what if” analysis not to have, but ok except for the fact that the system allows you to select a future date which results in a generic error. According to support team this is because the system doesn't allow future dates in forecasting, therefore if not supported, good UX design principals says you shouldn't allow users to set a future date.