[R] difftime result for days not an integer?

[Don MacQueen]

Which brings up another point.

The help page for difftime specifies that it operates on date-time or 
date objects. But
   '2004-08-05' is neither of these, it is a character object.
At this point, one might ask... I didn't give it what it asked for, 
what is it going to do?
(might give me an error message, might do who knows what, but find out!)

R is pretty good about automatic conversions between types, so it's 
reasonable that difftime()  would convert the arguments to a valid 
type, if it can. But which one, since there are two valid types?

And as we have seen, it is to date-time, not date.

For some, but certainly not all, R functions, details like this can 
be discovered by typing the name of the function at the R prompt, 
without the parentheses.

>  difftime
function (time1, time2, tz = "", units = c("auto", "secs", "mins",
     "hours", "days", "weeks"))
{
     time1 <- as.POSIXct(time1, tz = tz)
     time2 <- as.POSIXct(time2, tz = tz)
... etc ...

-Don

At 12:53 PM -0800 2/11/10, Thomas Lumley wrote:
>On Thu, 11 Feb 2010, Jonathan wrote:
>
>>Anybody have an idea why I would get a non-integer value for the
>>number of days here?
>>
>>>difftime('2004-08-05','2001-01-03',units='days')
>>Time difference of 1309.958 days
>
>Because it's not a whole number of 24-hour periods, due to daylight 
>saving time: 0.958 is 23/24.
>
>>
>>Would you just round off?
>>
>
>Yes, or use as.Date() if you only want to consider whole days
>
>R> as.Date('2004-08-05')-as.Date('2001-01-03')
>Time difference of 1310 days
>
>        -thomas
>
>Thomas Lumley			Assoc. Professor, Biostatistics
>tlumley at u.washington.edu	University of Washington, Seattle
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062