Lock today field afer save

Hi,

I have a  form with  java script. When the form is open the script fill the today date.

When the document is reopened with the date pre-filled, it dont change.

I put the code in the postsave event, but the date doesnt lock. 

Any Idea ?

if (this.getField ("DateDemand").value =="")

{

this.getField ("DateDemand").defaultValue = util.printd ("mm/dd/yyyy", new Date());

this.getField ("DateDemand").value = this.getField ("Today").defaultValue;

}

Regards

Pierre