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