Parameter Question - Filling other Parameters by Parameter Selection

I have a report that I am building that has to have a Fixed date parameter, as well as custom dates parameters.  The fixed dates are for Week to Date, Month to Date, and Year to Date.  I would like the selection in this drop down to auto-fill the"Start Date" and "End Date" parameters.

I have the logic written, however I cannot get the values to pass from the fixed date to the Start/ End dates.  I have tried setting them as available options in the parameter properties, however it changes it from a date field to a drop down. Is it possible to do this?

Here is a screenshot for more clarity: 

<url> http://s9.postimage.org/8cjttcu59/Untitled.jpg </url>

Thanks,
Brian