dateTime changes from POSIXct to factor when averaging - R -


i have following example:

begin_date = as.posixlt("1990-01-01", tz = "gmt") # 30 year dataset dat = data.frame(datetime = begin_date + (0:(24*30*20)) * (1800)) dat = within(dat,{speed = runif(length(datetime), 1, 10) })

i wish calculate hourly averages, use:

data <- aggregate(dat[colnames(dat)[2:length(colnames(dat))]],                   list(datetime = cut(dat$datetime,breaks = "hour")),                   mean,                   na.rm = true)

however, class of datetime column has changed posixct factor:

> class(dat$datetime) [1] "posixct" "posixt"  > class(data$datetime) [1] "factor"

why case, , how can change posixct?

 data$datetime <- as.posixct(as.character( data$datetime))

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