I suggest the following solution.

- set your default value for parameter to history (a date that you know, will never be used in this report, in my example it is set to 1.1.2007)

- add if statement to Select Expert:
If {?PDate}=date(2007,1,1) Then {Invoice.InvoiceDate} = CurrentDate else {Invoice.InvoiceDate} = {?PDate}

I have found this solution useful, although it would be easier, if you could just set value to CurrentDate.

Did you notice, that the suggested solution uses current date as a default, but also allows users to select their own date as a parameter?

By the way, BO just bought Crystal Reports and thus did not create it. Crystal Reports is actually quite flexible reporting program compared to its competitors, but if you have learned to use another one, it is harder to learn easier way to make things.