[R] Rounding in date.mdy from library(date)

[BXC (Bendix Carstensen)]

> The date library contains a function date.mdy that converts a number
> D to the date (month,date,year as a list) at D days after 1 Jan 1960.
> This a convention that fits in with SASs.
> 
> The logic would be that the result was the date at D days after
> 1 Jan 1960 00:00:00 (which is a POINT in time as opposed to a date which 
> is an interval), so that any D with 2<=D<3 was rounded to 3 Jan 1960
> and so on. This is because at any time D days (i.e. D*24h) after 
> 1.1.1960:00:00 with 2<=D<3 we would call the date "3 Jan 1960".
> 
> I cannot decipher the algorithm from the code in the function, but the 
> rounding procedure certainly produces some funny results, as the following
> 
> piece of clumsy code demonstrates:
> 
> dex <- function(jul.dat,N=1000)
>        {
>        # Functions that finds the fraction to add to a number
>        # to make the date.mdy round upwards (should be 1) 
>        (0:N)[(abs(diff(date.mdy(jul.dat+0:N/N)$day))>0.5)]/N
>        }
> jd <- 0:200 * 300
> rf <- 0
> for (i in 1:201) rf[i] <- dex(jd[i])
> plot(jd,rf,pch=".")
> 
> The obvous workaround would be to mask date.mdy by:
> 
> real.date.mdy <- function(jd) date.mdy(floor(jd))
> 
> Is this an error or is there a feature somewhere that I am missing?
> 
> Bendix Carstensen
> 
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