Mike,
Here's what you need to do. Extend the JDialog class.
You should have a private field called myDate, or whatever. Then you should have a set and get method for this field. The set method should be private, since I'm assuming that you would not want to make it possible for an outside class to modify this field. And the get method should be either protected or public, depending on which packages you want to have access to this dialog. When the dialog closes, you call the set method and set the date field internally, before calling dispose(). Since the dialog is modal, your calling class execution will stop and will wait on the JDialog to close. After the statement that calls the JDialog in your calling class, you call the get method from the JDialog and set your text field on the JFrame. Hope this helps!
Barry