How to initialize data.frame with column of type POSIXct?

You can try

df <- data.frame(a=numeric(), b=character(), c=as.POSIXct(character()))

Similarly, you can create a POSIXct column of NAs in a data frame with > 0 rows by creating a new column with as.POSIXct(NA).

An additional tip to the above initialization: If you begin rbind() activities to add rows to this empty data frame, you may encounter an error like the following if you follow this pattern:

oneDF <- rbind(oneDF,twoDF,stringsAsFactors=FALSE)
Error in as.POSIXct.default(value) :
  do not know how to convert 'value' to class "POSIXct"

I finally discovered that removing the stringsAsFactors=FALSE allowed for the POSIXct value (both integer time and time zone) to transfer to the target DF.

oneDF <- rbind(oneDF,twoDF)

examining the result:

unclass(oneDF$mytime)
[1] 1282089600
attr(,"tzone")
[1] "GMT"