library(lubridate)

Attaching package: ‘lubridate’

The following object is masked from ‘package:base’:

    date

> data$Last_follow_up_date <- mdy(data$Last_follow_up_date)
> class(data$Last_follow_up_date)
[1] "Date"
> parse_date_time("2016-08-31","ymd" )
[1] "2016-08-31 UTC"
> ifelse[as.interval(data$Last_follow_up_date,parse_date_time("2016-08-31","ymd" ))>0, data$Last_follow_up_date, data$Last_follow_up_date<- 2016-08-31]
Error in .setupMethodsTables(fdef, initialize = TRUE) :
  no slot of name "group" for this object of class "derivedDefaultMethod"