* ======================================================================= 
*  File:     age.SPS .
*  Date:     10-Feb-2005 .
*  Author:   Bruce Weaver, bweaver@lakeheadu.ca .
*  Notes:    Calculate AGE using date of birth (DOB) and another date.
* ======================================================================= .

DATA LIST LIST /id (f5.0) dob(date11).
BEGIN DATA.
1 8-Aug-1936
2 9-Aug-1937
3 26-Jan-1957
4 18-May-1960
5 21-Aug-1963
6 12-Aug-1965
END DATA.

numeric today (date11).
compute today = $time.
exe.

var width dob today(11).

* Compute AGE using CTIME.DAYS .
* CTIME.DAYS yields the number of days between two date variables.
* To convert days to years, divide by 365.25 (the .25 to account for leap years).

compute age = ctime.days(today-dob)/365.25.
exe.
list all.

* Unfortunately, there is a poor example of computing age in one
*  of the SPSS manuals.  It uses the YRMODA function, as follows.

COMPUTE age2 = ( YRMODA(XDATE.YEAR(today),XDATE.MONTH(today)
 ,XDATE.MDAY(today))-YRMODA(XDATE.YEAR(dob),XDATE.MONTH(dob)
 ,XDATE.MDAY(dob))) / 365.25 .
EXECUTE .

list all.

* As you can see, it yields essentially the same results, but is far 
*  more complicated than the CTIME.DAYS method.

* ------------------------------------------------------------- .
* NOTE:  I just discovered the DATEDIFF function in version 13.
* It allows you to compute age as follows:
* COMPUTE AGE = DATEDIFF(today,dob,"days") / 365.25.
* This is very similar to the CTIME.DAYS method shown above.
* For backward compatibility, I would stick with CTIME.DAYS.
* ------------------------------------------------------------- .


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