Firebird 3.0

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create table orders(
  dates date
);

INSERT INTO orders (dates)
SELECT  '2018-08-05 ' FROM RDB$DATABASE UNION ALL
SELECT  '2018-08-06' FROM RDB$DATABASE UNION ALL
SELECT  '2018-08-07' FROM RDB$DATABASE UNION ALL
SELECT  '2018-08-08' FROM RDB$DATABASE UNION ALL  
SELECT  '2018-08-09' FROM RDB$DATABASE;
5 rows affected

select * from orders;

DATES

2018-08-05

2018-08-06

2018-08-07

2018-08-08

2018-08-09

select DATEADD(day,7,date '2018-08-07') from orders;

DATEADD

2018-08-14

2018-08-14

2018-08-14

2018-08-14

2018-08-14

SELECT count(*) 
FROM orders 
WHERE dates BETWEEN '2018-08-07'  AND  DATEADD(day,7,date '2018-08-07');

COUNT

3