Calculate Fiscal Year - tsql

Question

I am generating a case statement that will be a master shared dataset for our report date ranges since we frequently use the same date ranges. Currently I have previous week, previous month, previous year, current week, current month and current year. I would like to add our fiscal year which is (10/1 to 9/30) as well but I haven't been able to come up with a pure SQL way to do it.
DECLARE #frequency as nvarchar(20);
SET #frequency = 'CURRENT MONTH'
SELECT
CASE #frequency
WHEN 'PREVIOUS MONTH' THEN CONVERT(DATE,DATEADD(m,DATEDIFF(m,0,GETDATE())-1,0))
WHEN 'CURRENT MONTH' THEN CONVERT(DATE,DATEADD(dd, 0, DATEDIFF(dd, 0, DATEADD(dd,-(DAY(GETDATE())-1),GETDATE()))))
WHEN 'CURRENT WEEK' THEN CONVERT(DATE,DATEADD(dd, 1, DATEDIFF(dd, 0,DATEADD(DAY, 1 - DATEPART(WEEKDAY, GETDATE()), CAST(GETDATE() AS DATETIME)))))
WHEN 'PREVIOUS WEEK' THEN CONVERT(DATE,DATEADD(dd, -6, DATEDIFF(dd, 0,DATEADD(DAY, 1 - DATEPART(WEEKDAY, GETDATE()), CAST(GETDATE() AS DATETIME)))))
WHEN 'CURRENT YEAR' THEN CONVERT(DATE, DATEADD(YEAR, DATEDIFF(YEAR, 0, GETDATE()), 0))
WHEN 'PREVIOUS YEAR' THEN CONVERT(DATE, DATEADD(YEAR, DATEDIFF(YEAR, 0, GETDATE()) - 1, 0))
END as [START DATE],
CASE #frequency
WHEN 'PREVIOUS MONTH' THEN CONVERT(DATE,DATEADD(dd, 0, DATEDIFF(dd, 0,DATEADD(ms,-2,DATEADD(month, DATEDIFF(month, 0, GETDATE()), 0)))))
WHEN 'CURRENT MONTH' THEN CONVERT(DATE,DATEADD(D, -1, DATEADD(MONTH, DATEDIFF(MONTH, '19000101', GETDATE()) + 1, '19000101')))
WHEN 'CURRENT WEEK' THEN CONVERT(DATE,DATEADD(dd, 1, DATEDIFF(dd, 0,DATEADD(DAY, 7 - DATEPART(WEEKDAY, GETDATE()), CAST(GETDATE() AS DATETIME)))))
WHEN 'PREVIOUS WEEK' THEN CONVERT(DATE,DATEADD(dd, -6, DATEDIFF(dd, 0,DATEADD(DAY, 7 - DATEPART(WEEKDAY, GETDATE()), CAST(GETDATE() AS DATETIME)))))
WHEN 'CURRENT YEAR' THEN CONVERT(DATE, DATEADD(ms, -2, DATEADD(YEAR, 0, DATEADD(YEAR, DATEDIFF(YEAR, 0, GETDATE()) + 1, 0))))
WHEN 'PREVIOUS YEAR' THEN CONVERT(DATE, DATEADD(ms, -2, DATEADD(YEAR, 0, DATEADD(YEAR, DATEDIFF(YEAR, 0, GETDATE()), 0))))
END as [END DATE]

Answer 1

I've found the simplest way to perform this logic is to shift your date by the offset. If 10/1/2018 belongs to FY 2018, subtract the 9 months to convert the date to 1/1/2018. Then get the year value.
YEAR(DATEADD(MONTH,-9,'2018-09-01')) = 2017
YEAR(DATEADD(MONTH,-9,'2019-09-01')) = 2018
YEAR(DATEADD(MONTH,-9,[YourDateField]))
With this, you can also calculate the Fiscal Month
MONTH(DATEADD( month, -9, '2018-09-01 )) = 12
My company use the Year the FY ends, so 10/1/2018 would be part of the FY closing 2019. For my arrangement, I would need to add 3 months instead of subtract to get the calculation to come out right.
This approach leverages date math exclusively so should be able to use an index in most situations.
To find the given day for the datepart you're looking for, try this:
SELECT CurrentWeek = DATEADD( WEEK, DATEDIFF( WEEK, '2018-01-01', GETDATE()), '2018-01-01' ),
CurrentMonth = DATEADD( MONTH, DATEDIFF( MONTH, '2018-01-01', GETDATE()), '2018-01-01' ),
CurrentYear = DATEADD( YEAR, DATEDIFF( YEAR, '2018-01-01', GETDATE()), '2018-01-01' )
;
By basing the calculation on a seed date, you don't need to truncate the smaller time values.

Answer 2

According to your calculations, this will work:
DECLARE #frequency as nvarchar(20);
SET #frequency = 'FINANCIAL YEAR'
SELECT
CASE #frequency
WHEN 'FINANCIAL YEAR' THEN CONVERT(date, DATEADD(month, -1,
DATEADD(year, -1, DATEADD(month, -8, DATEADD(month, 13-MONTH(GETDATE()),
DATEDIFF(month, 0, GETDATE()), 0))))))
END as [START DATE],
CASE #frequency
WHEN 'FINANCIAL YEAR' THEN CONVERT(date, DATEADD(ms, -2,
DATEADD(month, -8, DATEADD(month, 13-MONTH(GETDATE()), DATEADD(month,
DATEDIFF(month, 0, GETDATE()), 0)))))
END as [END DATE];
Play with the -8 in order to change the financial year - e.g. if the finacial year if from 2017-03 to 2018-03, then it will be -9.
I hope that's helpful!
P.S. I used April (from 2017-04 to 2018-04) as financial year