Entity SQL Using Imported Function

Question

Hi I am trying to use a stored procedure as an imported function in an entity sql statement

See my statement below, but this does not seem to work and I'm having a hard time finding out how imported functions are used in Entity SQL

SELECT 
Cp.Calendar_Reference
FROM 
Model.Calendar("CY", DATETIME'1999-01-01 00:0:00', DATETIME'2009-01-01 00:0:00')  AS Cp

(Model.Calendar returns a collection of the type Entities.Calendar_Period)

Please can someone tell me what I'm doing wrong

Thanks

K