Sunday 16 March 2014

Same day last year

Many companies - especially retailers - deal with comparisons of week, this week versus last week, etc.  Very often they will want to compare the performance of one day in a week versus the same day last year - the "Like-for-like" day.

I have seen some developers tie themselves up in knots trying to calculate things based on different rules and taking leap years into consideration.  But it is often much simpler than they realise!  No matter what year, leap year or otherwise, the same day of the week last year is exactly 364 days ago.

   364 = 52 x 7.

One thing that I can't guarantee with this calculation is that the same day last year will be in the same week number.  This is because of the fact that some years have 53 weeks and some have 52.  The ISO rules define week 1 as the week on which the first Thursday of the year falls.  Different organisations may have different rules about the week numbers, such as week 1 always starts on the 1st January, or even April 1st.  If you need to compare weeks based on week number, rather than the 364 rule (so how do you compare week 53?), then the best way to handle that is to have a calendar table that defines the values.

Stephen Redmond is author of QlikView Server and Publisher and the QlikView for Developer's Cookbook
He is CTO of CapricornVentis a QlikView Elite Partner. We are always looking for the right people to join our team.
Follow me on Twitter: @stephencredmond

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