R shiny modify reactive data frame - r

Question

I'm trying to build an app in which I can select the data file (input$dataset), then add a new datetime column formatting date and time previous columns to make plots with ggplot2.
I use 'within' that previously worked in batch scripts and in Rstudio. But now I get this error message:
no applicable method for 'within' applied to an object of class
"reactive"
How can I apply this method to a reactive object? Should I use another command? cbind? ddply?
datos=reactive({
read.csv(input$dataset,header=T,sep=";",na.strings="-99.900")
within(datos, datetime <- as.POSIXct(paste(FECHA,H_SOLAR),format = "%y/%m/%d %H:%M:%S"))
})
Thanks in advance
EDIT:
Following the answer below I understand a reactive source can't be modified, say add a column to the data frame. The point is I want to use ggplot in this way (adapting an old R script):
p=ggplot(datos(),aes_string(x="datetime", y=input$var,colour="as.character(stat_id)")) +
geom_line()
}
So, how should I add datetime to datos? Maybe creating datos2 as a new reactive source merging datos and datetime?
EDIT 2
Added full code to github https://github.com/pacomet/git

Answer 1

You cannot change the datafile directly - it is a reactive source that cannot be changed except by a user input (in this case the choice of data file).
You have 2 choices (that I know of):
1) Make a new object that holds the reformatted date:
NewDate<-reactive({ as.POSIXct(paste(FECHA,H_SOLAR),
format = "%y/%m/%d %H:%M:%S")})
then use NewDate() as your variable for graphing.
2) Change the date format within the function that makes the graph. e.g.
plot(x~as.POSIXct(paste(FECHA,H_SOLAR),format="%y/%m/%d %H:%M:%S"),
data=datos())
Here is a somewhat similar issue:Formatting reactive data.frame
EDIT
In response to the edited question - here is an updated answer.
I don't know much about ggplot but if the issue is to get this all into one data.frame, then you might want to do something like this:
datos=reactive({read.csv(input$dataset,header=T,sep=";",na.strings="-99.900"))}
NewDate<-reactive <- ({as.POSIXct(paste(FECHA,H_SOLAR),
format = "%y/%m/%d %H:%M:%S" )})
datos2<-reactive({ data.frame(datos(),NewDate() })
Then try using datos2() in ggplot - I think that should give you what you need.

Answer 2

I think this question can be closed thanks to the answer for #dieter-menne to another question about subsetting reactive data frames. The point is to create a new local variable, similar to #john-paul suggestion.
Please take a look at https://stackoverflow.com/a/20569512/709777