Data Validation For Datetime Parameter In SSRS

Hi,

I wanted to know more about validation of SSRS parameters. I have a simple report which has a parameter called startdate of DateTime datatype. The datetime parameter in SSRS takes manual input as well. So, the user can enter any junk value. I want to ensure that the input parameter is in correct format and I want to display an error msg when the format is incorrect. My report has the following VB code for validation:

Public Function Validate( ByVal startdate As String) As Boolean
If IsDate(startdate) = True Then
Return True
Else
Return False
End If
End Function

And my report has a textbox which has the expression property set to;

=Code.Validate(Parameters!startdate.Value)
the textbox on the report has to display if the entered date is valid or not.

But, when i enter an erroneous date, SSRS doesn't render the report and throws a generic error. This happens even before the code written for validating the parameter executes.

Also couldn't find a way to disable the manual input for the datetime parameter. Even that would solve the problem.

Another alternative was to make the startdate parameter as string, but i want the calendar control button to be provided for the user.