PFBC dynamic / conditional fields

I am using the PHP Form Builder Class for a form.

Basic idea: I need to set it up such that what you enter in one field is automatically entered into another field, but you can still change the second field.

More details: I have a start date and an end date. When you choose a start date, it should automatically populate the end date with that date. However, you can still choose a different end date if you wish.

I am not sure how to do this with the PFBC. So far I've found examples of using Javascript to hide a field or create a new field, but not to populate the field.

I am guessing my final result is going to need to be something with Javascript, but I would be open to any suggestions if anyone knows of this functionality being more easily obtained using PFBC.

Thanks!