str(user_latest_order_time)
chr [1:16383] "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "\\N" "44348.404988425929" "\\N" "\\N" ...
> x<-as.POSIXct(user__latest_order_time)
Error in as.POSIXlt.character(x, tz, ...) : 字符串的格式不够标准明确

不知道是不是因为数据中有\N的缘故。。