[R] Aggregate records to 10min

[Rolf Fankhauser]

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