Passing a date from a form to a form part where the datasets are not the same.

I have a form where the user enters a  date into a variable _reporthrudate .  This form has a form part that uses a different dataset but the dataset needs to calculate based on the _reportthru date that the user entered in the main form.   How do I pass the date entered by the user in the main form to the form part form so that my calculations as for the same periods.   The forms do not use the same dataset.

Thanks .