dynamic prompts in crystal reports 2008

I have a crystal report 2008 built out of stored procedure(Procedure is using SQL server 2008 database).

there is a datetime parameter in stored procedure.when it is coming to crystal reports it is showing as datetime but when i change the parameter type to dynamic the paramter datetype is changing to string from datetime, i am not able to change that since it was disabled.

I am changing that to dynamic because i would like to use LOV for that.

Any suggestions on this?

thanks!