Question

How do I go back to last April 4th (from today), minus 4 years, and return that date?

I could start by checking if today’s date is before or after April 4 this year and go from there, but I would be interested to see different approaches to this issue.

This will be part of a larger select statement, so a self-contained SELECT or a function would be the best solution.

Thanks!

Answer 1

I can’t find any neater way of doing this – I normally fall back on my tried and true DATEADD/DATEDIFF pair to do date calculations, but I can’t think of a way to take account of how far through the year we are neatly – so we get a CASE expression as well:

select DATEADD(year,
DATEDIFF(year,'20010101',CURRENT_TIMESTAMP)
- CASE WHEN DATEPART(month,CURRENT_TIMESTAMP) IN (1,2,3) THEN 1
    WHEN DATEPART(month,CURRENT_TIMESTAMP) = 4 AND
        DATEPART(day,CURRENT_TIMESTAMP) < 4 THEN 1
    ELSE 0 END
,'19970404')

For testing particular dates to check edges conditions:

declare @dt datetime
set @dt = '20120403'
select DATEADD(year,
DATEDIFF(year,'20010101',@dt)
- CASE WHEN DATEPART(month,@dt) IN (1,2,3) THEN 1
    WHEN DATEPART(month,@dt) = 4 AND
        DATEPART(day,@dt) < 4 THEN 1
    ELSE 0 END
,'19970404')

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If you just need 4th April of the year four years before the current year, then

DATEADD(year, DATEDIFF(year, '20010101', @dt), '19970404') 

will suffice.