R devel - Mar 2009 - Conversion and rounding of POSIXct

POSIXct/lt supports fractional seconds (see Sub-second Accuracy
section of man page), but there seem to be some inconsistencies in
their handling.

Converting to POSIXlt and back does not give back the same time for
times before the origin:
> t0 <- as.POSIXct('1934-01-05 23:59:59.00001')
> t0
[1] "1934-01-06 00:00:00 EST"      # rounding issue, see
below
> as.POSIXlt(t0)
[1] "1934-01-06 00:00:00 EST"
> as.POSIXct(as.POSIXlt(t0))
[1] "1934-01-06 00:00:01 EST"   # ???
> as.POSIXct(as.POSIXlt(t0)) - t0
Time difference of 1 secs

Also, POSIXct always rounds up when printing for times before the origin:
> as.POSIXct('1934-01-05 10:10:23')
[1] "1934-01-05 10:10:23 EST"
> as.POSIXct('1934-01-05 10:10:23.00001')
[1] "1934-01-05 10:10:24 EST"

and always rounds down when printing times after the origin:

as.POSIXct('2010-01-05 23:59:59.4')
[1] "2010-01-05 23:59:59 EST"
> as.POSIXct('2010-01-05 23:59:59.6')
[1] "2010-01-05 23:59:59 EST"
> as.POSIXct('2010-01-05 23:59:59.999')
[1] "2010-01-05 23:59:59 EST"

But the Description section says that POSIXct "represent[s] calendar
dates and times (to the nearest second)".  "Nearest" would seem
to
imply printing rounding-to-nearest, not rounding-up or rounding-down.

Stavros,

Two really quick comments:

a) you need to enable sub-second print formats
b) AFAIK pre-epoch times are second-class citizens

R> options("digits.secs"=6)   ## print with 6 digits for
microseconds
R> t0 <- as.POSIXct('1974-01-05 23:59:59.00001')
R> t0
[1] "1974-01-05 23:59:59.00001 CST"
R> as.POSIXlt(t0)
[1] "1974-01-05 23:59:59.00001 CST"
R> as.POSIXct(as.POSIXlt(t0)) - t0
Time difference of 0 secs

All that said, POSIXt is still under-documented and rather mysterious so I
won't / can't comment on all aspects of your post but the above should
shed
some light on the first few items.

Hth, Dirk

-- 
Three out of two people have difficulties with fractions.