Assuming that "EXTR0014" is Inventory and "a" is weight
"Weight =( Current member of Inventory(Time=02.02.2015)-Previous member of inventory( Time=01.01.2015))/100"
Use default.lgf:
*WHEN TV_EXTRACTION
*IS EXTR0014
*WHEN JOUR
*FOR %J%= %JOUR_SET%
*IS %J%
*REC(EXPRESSION=(%VALUE% -[JOUR].[TMVL(-1,%J%)])/100,TV_EXTRACTION =a)
*NEXT
*ENDWHEN
*ENDWHEN