Hi I would like to aggregate a rainfall series with 1min records (timestamp and value of 0.1mm from a tipping bucket raingauge) to 10min values by summing up the values. # ptime is a POSIXlt datetime value with tz="GMT" t10min <- 600*floor(as.integer(as.POSIXct(data$ptime))/600) w10min <- tapply(data$value, format(as.POSIXct(t10min, tz="GMT", origin = "1970-01-01"), "%Y-%m-%d %H:%M"), sum) write.table(as.matrix(w10min),"data 10min.txt", row.names=TRUE, col.names=FALSE, quote=FALSE) This code works but I would like to have the result in datetime format of %m/%d/%Y %H:%M. When I output this format the records are not chronologically sorted but text-sorted because dimnames of w10min is of type character (because of the format function). Is there an easier way summing up the records to 10min records? Thanks, Rolf