[R] Handling dates

[Göran Broström]

On Fri, Aug 19, 2005 at 01:35:45PM +0200, Peter Dalgaard wrote:
> Göran Broström <gb at stat.umu.se> writes:
> 
> > I have a problem with some functions handling dates, in packages 'date' and 
> > 'survival' (they seem to be identical). For instance, from the documentation,
> > 
> > --------------------
> > mdy.date {survival}	
> > 
> > R Documentation
> > 
> > Convert to Julian Dates
> > 
> > Description
> > 
> > Given a month, day, and year, returns the number of days since January 1, 1960.
> > Usage
> > 
> > mdy.date(month, day, year, nineteen = TRUE, fillday = FALSE,
> >          fillmonth = FALSE)
> > ----------------------------
> > but
> > 
> > > library(survival)
> > > mdy.date(12, 1, 1977)
> > [1] 1Dec77
> > 
> > Similar strange results appear in other date-related functions. I plan to 
> > write functions that converts, eg, "1977-01-31" to the real number 1977.084
> > and back. What function in  R  does what 'mdy.date' claims to do?
> 
> mdy.date does ...
> 
> > dput(mdy.date(12, 1, 1977))
> structure(6544, class = "date")

Thanks, I should have remembered that what functions print and what they 
return can be quite different things. This one returns "Julian dates".  
Which can be seen under "Value". But isn't the Description a bit misleading?
> 
> > mdy.date(1, 31, 1977)/365.2425+1960 # or /365.25 if you really want 1977.084
> [1] 1977.085

What I want is that the the fraction (0.084) should be equal to the fraction 
of the full year 1977 that is spent between 1977-01-01, at 00:00:00 and
1977-01-31, at noon. I know how to do it now.

Thanks again, Göran

> But with recent R's, as.Date is a better bet:
> 
> 
> > c(as.Date("1977-01-31") - as.Date("0000-01-01"))/365.2425
> [1] 1977.084
> 
> (notice that you need the c() to avoid the nonsense of
> 
> > (as.Date("1977-01-31") - as.Date("0000-01-01"))/365.2425
> Time difference of 1977.084 days
> 
> )
> 
> 
> -- 
>    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907