How to user Javascript UDF with web server control?

Hi everybody,
I have read an article on how to use javascript on ASP.NET Server controls at http://www.dotnetbips.com/displayarticle.aspx?id=8. javascript:alert('Hello') works absolutely fine. But how do I call a user defined function? E.g. I have a function called ValidDate() which I want to use to validate a date textbox. When I use
nDate.Attributes.Add("onchange", "javascript:ValidDate()")
it does'nt work. Please tell me how I can invoke my UDF.