从2月开始的日期增加6个月

Question

I am trying to subtract 3 months from February to a date using lubridate library, however, it always gives me date as 28. I know a lot of people have asked this question, but I have tried a lot of things, but still the same error. Looping through the day is an option. However, I was looking for something efficient

what my code does is this:

where test_end = "2019-02-28" and lastnmonth = 3

as.Date(test_end) %m-% months(lastnmonth)

I always get the output as "2018-11-28" instead of "2018-11-30"

Answer 1

lubridate::ymd("2019-02-28") - lubridate::period(3, "months")

解决蒙特问题的附加代码

lubridate::ceiling_date(lubridate::ymd("2019-02-28") - lubridate::period(3, "months"), "months") - lubridate::ddays(1)

Answer 2

You want the last of the month -- so you can use the trick that the next is a first, move that one by the number of months -- and then substract one day.

All in one line of base R:

R> seq(as.Date("2019-02-28") + 1, length=4, by="-1 month") - 1
[1] "2019-02-28" "2019-01-31" "2018-12-31" "2018-11-30"
R> 

We add one day, then use a sequence of four months (including current) backwards, and take out one day to be on the last day of the previous month.

If you just want one, simply add tail(..., 1) :

R> tail(seq(as.Date("2019-02-28") + 1, length=4, by="-1 month") - 1, 1)
[1] "2018-11-30"
R> 

Answer 3

Using Dirk's strategy we can draw-up a function to handle this type of operation:

start_date       <- lubridate::ymd("2019-02-28")
mths_to_subtract <- 3


deduct_mth <- function(date_object = start_date, 
                       mths_to_deduct = mths_to_subtract){

  days_to_ceiling_date <- lubridate::ceiling_date(date_object, "months") - date_object

  date_object + days_to_ceiling_date - lubridate::period(mths_to_deduct, "months") - days_to_ceiling_date

}

Gives the following answers:

> deduct_mth(ymd("2019-02-28"), 3)
[1] "2018-11-30"
> deduct_mth(ymd("2000-02-29"), 3)
[1] "1999-11-30"