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 Convert Timestamp to Number of Seconds (CEESECS) API. 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.