why months work in one case in R but not in another?

Question

As shown below I am trying to understand why it is NA in one case and not in another.

> x<-"2017-10-31"
> as.Date(x) - months(6)
[1] NA
> x<-"2018-07-31"
> as.Date(x) - months(6)
[1] "2018-01-31"
>

You can check https://stackoverflow.com/questions/43098932/r-subtracting-1-month-from-todays-date-gives-na i.e. `as.Date(x) %m-% months(6)# [1] "2017-04-30"` — akrun, Nov 14 '19 at 18:35

score 3 · Answer 1 · answered Nov 14 '19 at 18:38

That is because as.Date("2017-10-31") - months(6) (exactly) would be 2017-04-31, which doesn't exist.

If you want to subtract months without worrying about exceeding the last day of the month (moon calendar now!), use the fantastic functions %m+% and %m-% from lubridate package.

as.Date("2017-10-31") %m-% months(6)

[1] "2017-04-30"

For more details, ?lubridate::`%m+%`

score 2 · Answer 2 · answered Nov 14 '19 at 18:38

2

We can use %m- from lubridate to resolve the issue

library(lubridate)
as.Date(x) %m-% months(6)
# [1] "2017-04-30"

answered Nov 14 '19 at 18:38

akrun

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score 1 · Answer 3 · answered Nov 14 '19 at 18:37

1

Since there are only 30 days in April when you attempt to subtract 6 months from October 31st. It returns NA.

> x <- "2017-10-31"
> as.Date(x) - months(6)
[1] NA
> x <- "2017-10-30"
> as.Date(x) - months(6)
[1] "2017-04-30"

answered Nov 14 '19 at 18:37

Hansel Palencia

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why months work in one case in R but not in another?

3 Answers3