Small correction to Scott's code, he has the STUFF for 4 characters, this will wipe out both slashes. You could use either one below to get 'MM/YY':
declare @enddate datetime
set @enddate = getdate()
SELECT STUFF(CONVERT(varchar(10), DATEADD(DAY, -21, @enddate), 1), 3, 3, '')
SELECT STUFF(CONVERT(varchar(10), DATEADD(DAY, -21, @enddate), 1), 4, 3, '')
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