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
tlum...@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
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