[R] Need help on date calculation

[Gabor Grothendieck]

On Sat, Jul 10, 2010 at 3:07 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> On Sat, Jul 10, 2010 at 3:17 PM, Bogaso Christofer
> <bogaso.christofer at gmail.com> wrote:
>> Hi all, please see my code:
>>
>>
>>
>>> library(zoo)
>>
>>> a <- as.yearmon("March-2010", "%B-%Y")
>>
>>> b <- as.yearmon("May-2010", "%B-%Y")
>>
>>>
>>
>>> nn <- (b-a)*12   # number of months in between them
>>
>>> nn
>>
>> [1] 2
>>
>>> as.integer(nn)
>>
>> [1] 1
>>
>>
>>
>> What is the correct way to find the number of months between "a" and "b",
>> still retaining the INTEGER property?
>
> as.integer truncates so if its slightly below 2, as is the case here,
> you will get 1 as you observe so try any of these:
>
> # 1
> as.integer(round(nm))
>
> # 2 - just about any small number will do
> as.integer(nm + 0.001)
>
> Also see:
> http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>

# 3 - the mondate class in mondate package stores year/months
# as number of months since the instant between the end of 1999
# and start of 2000

library(mondate)
as.integer(mondate(b) - mondate(a))