I have two field in a SharePoint Form: Status & Closed Date.
Status is Choice coloumn which has two option Open & Close.
Closed Date is a Date Coloumn.
I want users to enter Closed Date when they choose Close in Status Field. It should validate the Closed date coloumn is filled once Close is choosen from Status column.

How to achieve this? I am new to JScript. I need to add this in Edit Form in SharePoint Designer 2007.

Thanks in advance.