Validation

Hi
I have two textbox of date fields a pop calender.
I need to check both the date fields to determine that the secnd date field should be greater than the first.
i have created a javascript validation function.
Validiate(): this is the javascript function that you will create for validating date(for checking both dates and compare if the second one isgreater than the first)
In asp .net or in code behind
-Textbox1.Attributes.Add("onChange","Validiate()"); is not working.
-In Asp.net for button ....onlicent click is also not working.
so  i tired for btncreate.Attributes.Add("onclick", "return validate()") inpage load.
Its working showing error msg if we select less date and click create button.But after that if we click proper date,still error message is not going for create button.
Can any one helps me .

...