I have a question regarding long tuple usage.

I'd like to use a long tuple format, but also perform a time level offset.

For example

*WHEN CATEGORY
*IS FORECAST
*WHEN TIME
*IS %CURRENT_PERIOD%
*WHEN ACCOUNT
*IS INSTALLBASE
*REC(EXPRESSION=(%VALUE% + ([ACCOUNT].[INSTALLBASE],[CATEGORY].[ACTUAL],[TIME].[%CURRENT_PERIOD-1%])/2),ACCOUNT=NEWACCOUNT)
*ENDWHEN
*ENDWHEN
*ENDWHEN

I've tried string manipulation like is used in MS, I've tried using TMVL on the time portion of the tuple, nothing has worked. Has anyone found a good way to manipulate a time member of a long tuple formula?

Thanks!