You can't do that with parameters in CR. However, one trick around this is to make the report a subreport of a dummy report. In the dummy report you can pass the datetime values to the parameters of the subreport (which is really your main report). It's a bit of a hack, but its the only way to get parameters to work the way you want them to.

I cover many tips and tricks for using parameters in Chapter 4 of my Encylcopedia book. You can find out more about my books at Amazon.com or reading the Crystal Reports eBooks online.