## Wednesday, 2 November 2011

### NOTE: Every Day's a Learning Day!

 Pope Gregory XIII
﻿The purpose of me attending SAS courses is to teach, and I'd like to think I'm successful at doing that, but not a course goes by without me learning a little tip or trick from one of the learners. Today's was most interesting...

We all know that SAS dates are stored as numbers, and the number represents the number of days since 1st January 1960 (or, number of days since my birthday plus 890 as I prefer to think of it!). Hence, 1st January 1960 is zero, and 2nd January 1960 is one.

A learner asked "what about dates before 1960, does SAS use negative numbers", to which I replied in the affirmative, but I then tried to enrich the answer by saying that you can only go back to 1548 because King Henry VIII changed the calendar at that time. Another learner politely pointed out that King Henry died in 1547, so I couldn't possibly be correct (my learners are well educated!).

Some quick research in the next break revealed some interesting information in the SAS 9.3 Language Reference: Concepts manual in the About SAS Date, Time, and Datetime Values section. Two things caught my eye:

1) SAS can perform calculations on dates ranging from A.D. 1582 to A.D. 19,900

2) SAS date values can reliably tell you what day of the week a particular day fell on as far back as September 1752, when the calendar was adjusted by dropping several days

So, (1) confirms that I got the year wrong, and it's long after King Henry's death, so I was proved utterly wrong! Further research turned-up an explanation of events in 1582. SAS Knowledge Base article 24808 by William Kreuter describes Calculating Age with Only One Line of Code but also mentions "Pope Gregory XIII proclaimed the Gregorian calendar in 1582" to properly deal with leap years. So there's the full explanation. Where I got my belief that it was 1548 and King Henry VIII I do not recall.

Item (2) was interesting for me too because I had not realised that SAS's day of the week capabilities only worked for a subset of the SAS date range.

So, every day's a learning day - teachers included!

#### 1 comment:

1. Andy, very interesting! For more insight into how SAS processes dates (including leap years), see http://blogs.sas.com/content/sasdummy/2010/04/05/in-the-year-9999/.