Time Series data with data every half hour

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I am working with data from the USGS with data every 30 minutes from
4/27/2011 to 8/25/2011.

I am having trouble with setting the frequency.

My R script is below:

shavers=read.csv("shavers.csv")

names(shavers)
  [1] "agency_cd"    "site_no"      "datetime"     "tz_cd"        "Temp"

  [6] "X04_00010_cd" "EC25"         "X05_00095_cd" "DO"
"X06_00300_cd"

[11] "pH"           "X07_00400_cd"

as.POSIXct(shavers[1,3])
[1] "2011-04-27 EDT"

as.POSIXct(shavers[nrow(shavers),3])
[1] "2011-08-25 23:00:00 EDT"

temp=ts(shavers[,5],as.POSIXct(shavers[1,3]),frequency=48)
Error in attr(data, "tsp")<- c(start, end, frequency) :

   invalid time series parameters specified

I have also used xts:

plot(temp)

library(xts)

temp=xts(shavers[,5],order.by=as.POSIXct(shavers[,3]),frequency=48)

stl(temp,s.window='periodic')
Error in stl(temp, s.window = "periodic") :

   series is not periodic or has less than two periods

The data has very clear daily periods.
... but you failed to tell this to the R functions correctly?
How can I configure my time
series to analyze this?
We do not know, since we do see a reproducible example you have been 
asked to provide in the posting guide.

Uwe Ligges

Thank you very much.

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