How To Create a NULL Date Field

I am working on an application that will help me manage my computer equipment - I have the inventory working fine and I decided to create a form that will help me manage trouble tickets.

This trouble tickets form is giving some issues - I currently have a Radio field with "Open" and "Closed" as it's options. On Add -> On Load will select the "Open" option and hide a Date_Closed field. On Add -> On Edit, checks if "Open" is selected and does the same if so.

Here's the tricky part - there are times when a ticket is closed, only to be re-opened because the issue is not fixed. So, since the ticket was Closed the "Closed" radio option is selected and the Date_Closed form is filled in.

I am trying to write a script for On Edit -> Validate, that will check if Status is set to "Open" if so - nullify the Date_Closed field. I really need this field to be blank so I can perform queries and see my open tickets. Right now, the Status may say Open - but there is still a Date_Closed value.

I've tried tons of different ways to go about this and I just can't figure it out in Deluge - any ideas?