First, SQL Server does not have a date type without a time, so we'll tweak it slightly:
Fortunately the format lends itself well to a quick conversion.
Create a SQL Expression containing:
convert(datetime, substring({table.datetimestring},1,11))
Now you can reference this field (I'll call it
{%CastedDate} in the record selection formula, as in:
{%CastedDate} >= {?MyStartDate}
Or however you intend to use the date parameters.
-k