How to solve: “Error in as.POSIXct.numeric(X[[2L]], …) : 'origin' must be supplied”

Question

we have created the following loop to plot the different parts of the data (with the package "latticeExtra").

library("latticeExtra")
All_dates<-unique(fad$Meet_date)
for (i in 1:11) { Date.i <- All_dates[i]
  layoutno <- unique(fad[fad$Meet_date == Date.i,28] )
  Data.i <- fad[fad$Meet_date == Date.i, ]
  Data.i$newtime <- as.POSIXct(Data.i$DateTime)
  prop <- with(Data.i,tapply(newtime,klasse, max))-with(Data.i,tapply(newtime,klasse, min))
png(paste(Date.i, ".png", sep = ""), height = 20, width = 30, units = "cm", res = 400)
dog <- xyplot(Ratio ~ Meet_times |as.factor(klasse), data = Data.i, main = paste(Date.i), 
layout = c(layoutno,1), scales = list(x = list(relation = "free",format  = "%H:%M")), 
xlab = "Time of measurement", ylab = "Ratio CH4/CO2")
print(resizePanels(dog,w = prop))
dev.off() }

However, we get this: Error in as.POSIXct.numeric(X[[2L]], ...) : 'origin' must be supplied..
I am a big noob with R, so I have no idea where this problem could lie.. My collegue also seems to be lost.. I know it is a big code, but I kinda hoped someone sees the problem rigthaway.. I tried several searches on this website and on Google, but this did not get me anything useful.

Please, help? Thanks a lot in advance!

Answer 1

Well, it's easy to get this problem to go away, but you need to be careful to get the date conversions right. The problem is that you are passing a number to as.POSIXct and not a Date object, and when you pass an integer, you need to give an origin date, so that it can compute the offset. For example:

> date<-as.Date("2013-10-18")
> as.POSIXct(date)
[1] "2013-10-17 20:00:00 EDT"
> date0<-unclass(date)
> date0
[1] 15996
> as.POSIXct(date0)
Error in as.POSIXct.numeric(date0) : 'origin' must be supplied

The real question is, how are you representing your dates in fad$Meet_date (and also why do you want to convert them to POSIXct). The correct origin and conversion will depend on what fad$Meet_date represents.