Add Javascript hook to Visual Form Builder dropdown list

Hi there,

I'm using the free version of the visual form builder application which seems excellent. However, I have a need for people to be able to request a booking time in the GMT time-zone when they may well be in a completely different time-zone.

So, what I'd like to do, is present the user with their corresponding local time of the appointment they have booked in GMT.

I've already put some code in to a description field along with a button definition which sort of works but interferes with the submit as it's contained within a form so that seems (a) too manual and (b) untidy

I have the appropriate code figured out calculate the TZ offset and adjust the date correctly but I cannot see how to add a hook to the dropdowns in Visual Form Builder. Ideally I want to catch the onchange event so that changing the hrs/minutes automatically updates a text field with the appropriate appointment time.

Thoughts please?