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How the datediff determinines the difference between two dates?

  • January 19, 2015 at 9:46 am

    #296788

    select datediff(wk, '2015-01-11', '2015-01-19') returns 1 (shouldn't it return 2?)

    select datediff(wk, '2015-01-10', '2015-01-19') returns 2

    Thanks

  • January 19, 2015 at 9:54 am

    #1771172

    I believe the difference in those calculations will use SQL Server's numbered weeks; this will change based on your DATEFIRST settings, but, assuming a DATEFIRST of Monday:

    1/11/2015 is one full week before 1/19/2015; you can see the basis for this by running a SELECT DATEPART(WEEK,'1/11/2015'), which should return a week number of 3.

    On the other hand, 1/10/2015 is two full weeks before 1/19/2015; again, a SELECT DATEPART(WEEK,'1/10/2015') will show as much, with a week number of 2.

    Hence, the difference between today, which is in week number 4, and 1/10/2015's week, which is 2, is 2. For 1/11/2015, the difference is 4 and 3, which is one.

    - 😀

  • January 19, 2015 at 10:12 am

    #1771175

    sonchoix (1/19/2015)

    select datediff(wk, '2015-01-11', '2015-01-19') returns 1 (shouldn't it return 2?)

    select datediff(wk, '2015-01-10', '2015-01-19') returns 2

    Thanks

    As per BoL: "DATEDIFF ... Returns the count (signed integer) of the specified datepart boundaries crossed between the specified startdate and enddate"

    Week boundary will depend on DATFIRST setting (default - 7 - Sunday for U.S. English)

    So, datediff will count a week for each crossing of week boundary starting from startday (if startday is a boundary day, it's not counted as there is no crossing of 23:59:59 to 00:00:00...)

    Your first pair has one crossing, therefore DATEDIFF returns 1,

    the second one has two corssings, hence 2 is returned.

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  • January 19, 2015 at 10:56 am

    #1771184

    Thank you guys

  • January 19, 2015 at 11:50 am

    #1771192

    Be advised that the "wk" DATEPART can change depending on language settings and DATEFIRST settings. It's not considered to be a good thing to use.

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  • January 20, 2015 at 3:33 am

    #1771354

    Jeff Moden (1/19/2015)

    Be advised that the "wk" DATEPART can change depending on language settings and DATEFIRST settings. It's not considered to be a good thing to use.

    +1

    I would calculate the difference in days and get the modulo of 7.

    (which might be a total different interpretation of difference in weeks)

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