Date Parameters in SSRS with wide range of Options

Hey There,

I have a requirement that, There should be a date type parameter with drop down list of options Current Date, Current Week,Previous Week,Custom.

Start Date parameter, If I select above options, date should get populated automatically. But when I select Custom,DatePicker UI should appear and allow me to select manually.

End Date parameter, If I select above options, date should get populated automatically. But when I select Custom,DatePicker UI should appear and allow me to select manually.

I was able to get the dates populated in Start Date and End Date textBoxes. 

For StartDate

=Switch(
Parameters!DateType.Value = "Current Date",Today,
Parameters!DateType.Value = "Current Week",DateAdd("d", 1 - DatePart(DateInterval.WeekDay, Today,FirstDayOfWeek.Monday), Today),
Parameters!DateType.Value = "Previous Week",DateAdd(DateInterval.Day, -6,DateAdd(DateInterval.Day, 1-Weekday(today),Today)))

For EndDate,

=Switch(
Parameters!DateType.Value = "Current Date",Today,
Parameters!DateType.Value = "Current Week",Today,
Parameters!DateType.Value = "Previous Week",DateAdd(DateInterval.Day, -0,DateAdd(DateInterval.Day, 1-Weekday(today),Today)))

But When I select Custom, I am not getting Date Picker UI. What should I do to get Datepicker . It is urgent. Please help me.