I wanted users to use the standard date-picker, but there is no way to format the date inside the query.  My work-around is a classic Reporting Services pattern – create an internal parameter that does the formatting for you.  The format string for VBScript is “yyyy-MM-ddTHH:mm:ss”.

Below is a screenshot of the Report Parameters dialog. I used the following code to just grab today’s date (no time).

     =CDate(CDate(Now()).ToString("MM/dd/yyyy"))

For the second parameter, I create an internal parameter that is dependent on the first.  This will prevent it from showing on the report and it will not be filled in until the start date is chosen.  Default value:

     =CDate(Parameters!StartDate.Value).ToString("yyyy-MM-ddTHH:mm:ss")

I created a test report to show how this works.  Here is the report definition:

Below is the report preview:  You can now use the parameter in your CAML query.

One other note on CAML date fields.  The time is ignored by default.  If you want to include the time, you must specify another attribute.  Example:

<Geq>

<FieldRef Name="LastModified" />

<Value Type="DateTime" IncludeTimeValue="TRUE">@LastModified!</Value>

</Geq>

I found a helpful post on CAML & DateTime that might be useful for you too.