You might use SUBDUR consecutively to get a value for *DAYS, *HOURS and *MINUTES. The number of minutes is then ((Days*1440)+(Hours*60)+Minutes). You might need to do some testing to determine what happens if the difference is greater than a whole Month.
Alternatively, you can convert two timestamps to two NumberOfSeconds values with the <a href=”http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=/apis/CEESECS.htm”>Convert Timestamp to Number of Seconds (CEESECS) API</a>. The difference between the two values is the number of elapsed seconds. Divide by 60 to obtain Minutes.
I’d probably go with the CEESECS API. A pretty straightforward proc could be created for this.