I use INTNX to track product sales by week, for example, where it is very efficient to code sales date into a week number. I would not use INTNX to calculate someone's age, because the function is using calendar dates, like Jan 1st, etc. for the interval boundaries. Also, it is worth doing testing to understand INTNX especially for "week", because Jan. 2, for example, may not give you the value you thought, depending on which year it is, and what day of the week Jan. 1st falls on. As for PROC SQL - I love PROC SQL, but that doesn't mean it's the right tool for everything you do. ("When all you have is a hammer, everything looks like a nail.") I prefer a DATA step for SAS functions, like the date functions.
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