Creating parameter based report in SSRS

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ASP.NET

I have a report with 3 parameters as follows:
1. Duration -> It can have values as Today, Yesterday,Last Month
2. Start Date
3. End Date

My requirement is, based on the option selected in Duration parameter, StartDate & EndDate
should get populated as well as there should be a provision for the user to enter these 2
dates.

I have written 2 VB functions in Code tab to as SetStartDate and SetEndDate that accepts
Duration as parameter and sets the dates accordingly.

I am calling my functions as follows by setting the Default Value and Available Value
expressions for Start Date and End Date:

=cdate(code.SetStartDate(Parameters!Duration.Value))


This is working fine, but it makes my datetime parameters readonly, so user is not able
to enter date of his choice.

How to achieve it?

My SP accepts Start Date & End Date as parameters.