How to subset a large data frame (ffdf) in R by da

I am trying to subset an FFDF by a date. Below, I have successfully created such a subset using a normal data frame. But I needed some help in applying this to an FFDF. My attempt, along with the error message, is listed in the code comment. Many Thanks in advance!

#Create a normal data frame (in production this is read directly into an ffdf 
#through a csv file)

start  <- c("01/01/2010", "01/01/2011", "01/01/2012", "01/01/2012", "01/01/2012")
end  <- c("31/12/2010", "31/12/2011", "31/12/2012", "31/12/2012", "31/12/2012")
amount <- c(10,20,30,40,50)
df <- data.frame(start,end,amount)

#Ensure subsetting works on a normal data frame

  #convert type to proper date (this has to be done in production after csv file
  #has been read in)
  df$start <- as.Date(df$start, format="%d/%m/%Y")
  df$end <- as.Date(df$end, format="%d/%m/%Y")

  #Subset
  df <- subset(df, start == as.Date("2012-01-01",format="%Y-%m-%d"))

  #Works :) Now let's try with ffdf

ffdf <- as.ffdf(df)

  #Type conversion for dates (again, applied in production after mammoth csv has
  #been read in)
  ffdf$start <- as.Date(ffdf$start, format="%m/%d/%Y")
  ffdf$end <- as.Date(ffdf$end, format="%m/%d/%Y")

  #Subset
  ffdf <- subset.ff(ffdf, start==as.Date("2012-01-01",format="%Y-%m-%d"))
  #ERROR: Error in ffdf(x = x) : ffdf components must be atomic ff objects

Use subset.ffdf from package ffbase. Subset is a generic function in R, and ffbase implements it for ffdf objects. So you can just use subset as you would do with a regular data frame.

df <- data.frame(start=c("01/01/2010", "01/01/2011", "01/01/2012", "01/01/2012", "01/01/2012"),end=c("31/12/2010", "31/12/2011", "31/12/2012", "31/12/2012", "31/12/2012"),amount=c(10,20,30,40,50))
df$start <- as.Date(df$start, "%d/%m/%Y")
df$end<- as.Date(df$end, "%d/%m/%Y")

require(ffbase)
myffdf <- as.ffdf(df)
test <- subset(myffdf , start==as.Date("2012-01-01",format="%Y-%m-%d"))
test