Re: managing dates in POSIXct format at 00

R does this for convenience when printing values. But it's not always
convenient when writing to a file. Instead, you could use strftime to
convert to your desired character format before writing:

mydf$data_POSIX <- strftime(mydf$data_POSIX, "%Y-%m-%d %H:%M:%OS3")

you can optionally change the 3 to a number between 0 and 6. If you choose
0, it's easier to just use %S instead.


On Fri, Nov 14, 2025, 12:30 Stefano Sofia via R-help <[email protected]>
wrote:

> Dear R-list users,
>
> this question will seem silly to most of you, but I really got mad trying
> to find a solution with no success, and therefore I decided to write here.
>
>
> Suppose I create the following data frame:
>
>
> mydf <- data.frame(date = c("2007-11-19 00:00:00.000", "2007-11-19
> 06:00:00.000", "2007-11-19 12:00:00.000", "2007-11-19 18:00:00.000",
> "2007-11-20 00:00:00.000"), value = rep(10, 5))
>
> mydf$data_POSIX <- as.POSIXct(mydf$date, format = "%Y-%m-%d %H:%M:%S",
> tz="Etc/GMT-1")
>
> max(mydf$data_POSIX)
>
>
> gives, obviously,
>
>
> [1] "2007-11-20 +01"
>
>
> Now if I export this data frame in a txt file
>
>
> write.table(mydf, file="mydf.txt", sep = ",", dec=".", row.names=FALSE,
> col.names = TRUE, quote=FALSE)
>
>
> the file is stored like
>
>
> date,value,data_POSIX
> 2007-11-19 00:00:00.000,10,2007-11-19
> 2007-11-19 06:00:00.000,10,2007-11-19 06:00:00
> 2007-11-19 12:00:00.000,10,2007-11-19 12:00:00
> 2007-11-19 18:00:00.000,10,2007-11-19 18:00:00
> 2007-11-20 00:00:00.000,10,2007-11-20
>
> Hours, minutes and seconds at 00 are not shown anymore.
> When I read back this file into R:
>
> mydf2 <- read.table(file="mydf.txt", header = TRUE, sep=",", dec = ".")
>
> mydf2$data_POSIX <- as.POSIXct(mydf2$data_POSIX, format = "%Y-%m-%d
> %H:%M:%S", tz="Etc/GMT-1")
>
> I get
>                      date value          data_POSIX
> 1 2007-11-19 00:00:00.000    10                <NA>
> 2 2007-11-19 06:00:00.000    10 2007-11-19 06:00:00
> 3 2007-11-19 12:00:00.000    10 2007-11-19 12:00:00
> 4 2007-11-19 18:00:00.000    10 2007-11-19 18:00:00
> 5 2007-11-20 00:00:00.000    10                <NA>
>
> and
>
> max(mydf2$data_POSIX)
>
>
> does not work anymore.
>
> Where is my mistake? Is there a solution?
>
> Thank you for your attention and your help
>
> Stefano
>
>
>
>
>
>          (oo)
> --oOO--( )--OOo--------------------------------------
> Stefano Sofia MSc, PhD
> Civil Protection Department - Marche Region - Italy
> Meteo Section
> Snow Section
> Via Colle Ameno 5
> 60126 Torrette di Ancona, Ancona (AN)
> Uff: +39 071 806 7743
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