At first I thought there was a bug in SAS!  Later I realized it was a problem with one of my "if" conditions.  It was actually an issue of me not realizing that '01JAN1960' has a value of 0.   Let's say I want to calculate the age for some records, given a birthdate field that is usually missing. Here is a sample data set: data sample; id='1';birthdate='01JAN1970'd; output; id='2';birthdate='02JAN1970'd;output; id='3';birthdate=.;output; id='4';birthdate='01JAN1960'd;output; run; /*In a second data step, I calculate the ages:*/ data ages; set sample; if birthdate then age = floor((intck('month',birthdate,today()) - (day(today()) < day(birthdate))) / 12); run;   Do you see the problem?  The intention is to calculate the age only if the birthdate is not missing.  However, if the birthdate is on January 1, 1960, then the value is 0, which is a numeric false.  The "if" resolves to false, so the "then" part of the statement doesn't get executed.   The "if" statement should read as follows:   if birthdate ne . then age = floor((intck('month',birthdate,today()) - (day(today()) < day(birthdate))) / 12);   It was one of those gotcha moments. ... View more