My point is this: if we do temp<-tDates aggregate(DF[,c('V2', 'V3')], list(format(temp, "%H:%M:00")), FUN = sum) Does agrregate still know it's refering to DF$V1 - and if so- how? -- View this message in context: http://n4.nabble.com/Group-by-tp1588694p1589600.html Sent from the R help mailing list archive at Nabble.com.