Formula as parameter field problem

Hi All

I am using crystal xi and have a main report with a subreport. Both reports have a 'datetime' field that I need to link on and create a parameter field.

For end user ease I have changed the 'datetime' fields to 'date' and linked the subreport using that formula, and all works well

I set up a parameter field for a Date Range and set the {@date} field from Select Expert = paramater field and now the information has changed.

I have manually checked the information returning and can't find any obvious problem.

Any theories??

Thanks very much in advance.