var startDate = new SqlParameter("StartDate", System.Data.SqlDbType.DateTime) { Value = "2013-10-20T01:00:00.000" };
var endDate = new SqlParameter("EndDate", System.Data.SqlDbType.DateTime) { Value = "2013-10-28T01:00:00.000" };

var result = _context.Database.SqlQuery<MyEntity>("dbo.MyStoredProcedure @StartDate, @EndDate",
                startDate, endDate).AsQueryable();

var result =
                _context.Database.SqlQuery<MyEntity>("Exec dbo.MyStoredProcedure @StartDate = {0}, @EndDate = {1}",
                    "2013-10-20T01:00:00.000", "2013-10-28T01:00:00.000");