SQL Server 2014

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create table your_table (
 id int identity primary key, 
 BillingDateFrom datetime, 
 BillingDateTo as (dateadd(day,(7),[BillingDateFrom]))
)
insert into your_table (BillingDateFrom) values
('2021-02-21 00:00')
1 rows affected

ALTER TABLE dbo.your_table
 ADD BillingDealerPeriodID
  AS (FORMAT(dateadd(day,7,[BillingDateFrom]), 'MMyy'));
select *
from your_table


id
BillingDateFrom
BillingDateTo
BillingDealerPeriodID


1
2021-02-21 00:00:00.000
2021-02-28 00:00:00.000
0221