'-' calls 'difftime' which, if you don't specify the units, makes the
following assumptions in the code:
> 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)
z <- unclass(time1) - unclass(time2)
units <- match.arg(units)
if (units == "auto") {
if (all(is.na(z)))
units <- "secs"
else {
zz <- min(abs(z), na.rm = TRUE)
if (is.na(zz) || zz < 60)
units <- "secs"
else if (zz < 3600)
units <- "mins"
else if (zz < 86400)
units <- "hours"
else units <- "days"
}
}
switch(units, secs = structure(z, units = "secs", class = "difftime"),
mins = structure(z/60, units = "mins", class = "difftime"),
hours = structure(z/3600, units = "hours", class = "difftime"),
days = structure(z/86400, units = "days", class = "difftime"),
weeks = structure(z/(7 * 86400), units = "weeks", class = "difftime"))
}
You can use difftime explicitly so you can control the units.
> c(as.POSIXct('2009-09-01'), as.POSIXct('2009-10-11')) -
> as.POSIXct('2009-08-31')
Time differences in days
[1] 1 41
> difftime(c(as.POSIXct('2009-09-01'), as.POSIXct('2009-10-11')),
> as.POSIXct('2009-08-31'), units='sec')
Time differences in secs
[1] 86400 3542400
>
>
On Fri, Sep 11, 2009 at 7:50 AM, Heinz Tuechler <tuech...@gmx.at> wrote:
> Dear All,
>
> what determines if a difference between POSIXct objects gets expressed in
> days or seconds?
> In the following example, it's sometimes seconds, sometimes days.
>
> as.POSIXct('2009-09-01') - as.POSIXct(NA)
> Time difference of NA secs
>
> c(as.POSIXct('2009-09-01'), as.POSIXct(NA)) -
> c(as.POSIXct('2009-09-01'), as.POSIXct('2009-08-31'))
> Time differences in secs
> [1] 0 NA
>
> c(as.POSIXct('2009-09-01'), as.POSIXct(NA)) -
> as.POSIXct('2009-08-31')
> Time differences in days
> [1] 1 NA
>
> Thanks,
> Heinz
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
--
Jim Holtman
Cincinnati, OH
+1 513 646 9390
What is the problem that you are trying to solve?
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