aggregate semi-hourly data not 00-24 but 9-9

Hi Stefano,
If you mean from 9am on one day to 9am on the following day, you can
do a trick. Simply subtract 9hrs from each timestamp and then you want
midnight to midnight for these adjusted times, which you can get using
the method you followed.

I googled and found that lubridate::hours() can be used to add or
subtract hours from a POSIXct.

library(lubridate)

day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M",
tz="Etc/GMT-1")
day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M",
tz="Etc/GMT-1")
df1 <- data.frame(data_POSIX=seq(day_1, day_2, by="30 min"))
df1$hs <- rnorm(nrow(df1), 40, 10)
df1$diff[2:nrow(df1)] <- diff(df1$hs)

df1$data_POSIXminus9 <- df1$data_POSIX - lubridate::hours(9)
df1$dayX <- format(df1$data_POSIXminus9,"%y-%m-%d")
df2X <- aggregate(diff ~ dayX, df1, sum)
df2X

HTH,
Eric

On Mon, Sep 21, 2020 at 5:30 PM Stefano Sofia
<stefano.sofia at regione.marche.it> wrote:

Dear R-list members,
I have semi-hourly snowfall data.
I should sum the semi-hourly increments (only the positive ones, but

this is not described in my example) day by day, not from 00 to 24 but
from 9 to 9.

I am able to use the diff function, create a list of days and use the

function aggregate, but it works only from 0 to 24. Any suggestion for
an efficient way to do it?

Here my code:
day_1 <- as.POSIXct("2020-02-19-00-00", format="%Y-%m-%d-%H-%M",

tz="Etc/GMT-1")

day_2 <- as.POSIXct("2020-02-24-12-00", format="%Y-%m-%d-%H-%M",

tz="Etc/GMT-1")

df1 <- data.frame(data_POSIX=seq(day_1, day_2, by="30 min"))
df1$hs <- rnorm(nrows(df1), 40, 10)
df1$diff[2:nrow(df1)] <- diff(df1$hs)
df1$day <- format(df$data_POSIX,"%y-%m-%d")
df2 <- aggregate(diff ~ day, df, sum)

Thank you for your help
Stefano

         (oo)
--oOO--( )--OOo----------------
Stefano Sofia PhD
Civil Protection - Marche Region
Meteo Section
Snow Section
Via del Colle Ameno 5
60126 Torrette di Ancona, Ancona
Uff: 071 806 7743
E-mail: stefano.sofia at regione.marche.it
---Oo---------oO----------------

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