AOT Query set datetime range

In AX 2012, I have an AOT query that loads ledger entries filtered by postedDate.  I am using SSRS report to display the result.

What I have done is:

1) I add a range in the query based on the PostedDate

2) on the report, I created 2 report parameters called startDate and endDate.

3) I used a data method to create a string with format "startDate..endDate" (e.g. 6/1/2012..9/1/2012), then I assign the string to PostedDate parameter hoping it will filter the query by the PostedDate using start/end date.

The report runs ok  inside Visual Studio but it gives error on the client:

Report parameter date time default value 6/1/2012..9/1/2012 is not in the expected format. Please eidt report to provide default value in cluture invariant mm/dd/yyyy

Can anyone help?

Thanks

Holly