lm works with difftime objects but then if you try to get the summary it fails with an error: fit <- lm(as.difftime(Time, units = "mins") ~ demand, BOD) summary(fit) ## Error in Ops.difftime((f - mean(f)), 2) : ## '^' not defined for "difftime" objects A number of other lm methods also fail, e.g. plot(fit), but others work, e.g. coef(fit), resid(fit) -- Statistics & Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com