Multiply?
Can't you get the duration of the difference in seconds, divvy that up, and then repeatedly add it to the previous result? -- Free map of local environmental resources: http://CambridgeMA.GreenMap.org -- MOTD on Prickle-Prickle, the 73rd of Confusion, in the YOLD 3173: Any damn fool can making sweeping generalizations about damn fools.
