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In learning how to get the date of a day of the week from the previous week (e.g. the date of Monday from the previous week), I found the following two expressions coming up commonly, where you change the very last number (e.g. 0=Monday) depending on which day you want:

SELECT DATEADD(wk, DATEDIFF(wk, 6, GETDATE()), 0)

SELECT DATEADD(wk, DATEDIFF(wk, 7, GETDATE()), 0)

Both seem to return the correct Date 2017-05-29 when I run it today (2017-06-17).

My question is - how does this work? It seems to be getting a count of weeks since 6 or 7 weeks after 1900-01-01? And why does it work with both a 6 and 7?

Also, are there edge cases one would need to be aware of where it might not give the expected results?

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This takes advantage of the fact that 1900-01-01 is a Monday. So adding n number of weeks will also be a Monday.

Here are the datetime values that are being represented by the integers in that expression:

dateadd(week, datediff(week, 7 /*'19000108'*/, getdate()), 0 /*'19000101'*/)

Adding the number of weeks since 1900-01-08 (the week after the first Monday) to 1900-01-01 returns one week less than the current week, i.e. Monday of last week.

These three are equivalent: 

select dateadd(week, datediff(week, 7 , getdate()), 0)
select dateadd(week, datediff(week, '19000108', getdate()), '19000101')
select dateadd(week, datediff(week, 0, getdate())-1, 0)

rextester demo: http://rextester.com/BQX59903

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  • This all makes sense except 1 thing... the 6 and the 7 - why would one add either 6 or 7 weeks from the first monday? I suspect they used 6/7 thinking aout "days", but it's not days, its wk right? – GWR Jun 7 '17 at 13:22
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    @GWR 6 or 7 are 1900-01-07 and 1900-01-08 respectively, not weeks. The likely cause of someone using 7 would be if their native calendar week starts on a Monday, vs US where it starts on Sunday. The reason 6 works even if someone has set datefirst 1 (to set the first day of the week as Monday instead of Sunday) explicitly or implicitly by language setting, datediff(week...) is going to be performed as if set datefirst 7 was in place. rextester demo: rextester.com/YWJRZ24690 – SqlZim Jun 7 '17 at 13:32
  • I get it... I was mixing it up... the 6 and 7 I was thinking of as count of days or weeks, but its a value that resolves into a date expression... Thanks for the assist. – GWR Jun 7 '17 at 13:41
  • @GWR Happy to help! – SqlZim Jun 7 '17 at 13:41

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