Hi, I have two data sources. One records time in PST time zone, the other in GMT. I want to compute the difference between the two, but don't see how. Here is an example where I compute time difference between identical times each (meant to be) relative to its time zone.> as.POSIXlt("2000-05-10 10:15:00", "PST") - as.POSIXlt("2000-05-1010:15:00", "GMT") Time difference of 0 secs I was expecting to see 8hrs (which is the time difference between London and San-Francisco). Why is it so and what is the correct way of doing it? I use R-1.8.1 on RH-7.3. Thanks, Vadim [[alternative HTML version deleted]]
On Fri, Apr 23, 2004 at 11:30:19AM -0700, Vadim Ogranovich wrote:> Hi, > > I have two data sources. One records time in PST time zone, the other in > GMT. I want to compute the difference between the two, but don't see > how. Here is an example where I compute time difference between > identical times each (meant to be) relative to its time zone. > > > as.POSIXlt("2000-05-10 10:15:00", "PST") - as.POSIXlt("2000-05-10 > 10:15:00", "GMT") > Time difference of 0 secs > > I was expecting to see 8hrs (which is the time difference between London > and San-Francisco). Why is it so and what is the correct way of doing > it?Seems to work with POSIXct in 1.8.1 and 1.9.0:> as.POSIXct("2000-05-10 10:15:00", "PST8PDT") - as.POSIXct("2000-05-1010:15:00", tz="UTC") Time difference of 7 hours Dirk> > > I use R-1.8.1 on RH-7.3. > > Thanks, > Vadim > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://www.stat.math.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html >-- The relationship between the computed price and reality is as yet unknown. -- From the pac(8) manual page
Thank you for the lead, Dirk! Indeed this works on my machine too:> as.POSIXct("2000-05-10 10:15:00", tz="PST8PDT") -as.POSIXct("2000-05-10 10:15:00", tz="GMT") Time difference of 7 hours However when I replace POSIXct by POSIXlt it breaks (this looks like a bug to me):> as.POSIXlt("2000-05-10 10:15:00", tz="PST8PDT") -as.POSIXlt("2000-05-10 10:15:00", tz="GMT") Time difference of 0 secs Now a couple of new questions: * how could I learn about appropriate names for time zones? For example I was using "PST" whereas it seems I had to use either "PST8" or "PST8PDT". Why "PST" was not good? Is it documented anywhere? * there seems to be no difference betweeen GMT and BST on my machine though PST8 and PST8PDT are treated properly: # PST8 is not identical to PST8PDT> ISOdatetime(2003, seq(12), 1, 10, 0, 0, tz="PST8") - ISOdatetime(2003,seq(12), 1, 10, 0, 0, tz="PST8PDT") Time differences of 0, 0, 0, 0, 3600, 3600, 3600, 3600, 3600, 3600, 0, 0 secs # GMT0 is identical to BST> ISOdatetime(2003, seq(12), 1, 10, 0, 0, tz="GMT0") - ISOdatetime(2003,seq(12), 1, 10, 0, 0, tz="BST") Time differences of 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 secs Why is such dichotomy? Thanks, Vadim> -----Original Message----- > From: Dirk Eddelbuettel [mailto:edd at debian.org] > Sent: Friday, April 23, 2004 1:52 PM > To: Vadim Ogranovich > Cc: R-Help > Subject: Re: [R] time zones in POSIXt > > > > On Fri, Apr 23, 2004 at 11:30:19AM -0700, Vadim Ogranovich wrote: > > Hi, > > > > I have two data sources. One records time in PST time zone, > the other > > in GMT. I want to compute the difference between the two, but don't > > see how. Here is an example where I compute time difference between > > identical times each (meant to be) relative to its time zone. > > > > > as.POSIXlt("2000-05-10 10:15:00", "PST") - > as.POSIXlt("2000-05-10 > > 10:15:00", "GMT") > > Time difference of 0 secs > > > > I was expecting to see 8hrs (which is the time difference between > > London and San-Francisco). Why is it so and what is the > correct way of > > doing it? > > Seems to work with POSIXct in 1.8.1 and 1.9.0: > > > as.POSIXct("2000-05-10 10:15:00", "PST8PDT") - > > as.POSIXct("2000-05-10 > 10:15:00", tz="UTC") > Time difference of 7 hours > > > Dirk > > > > > > > I use R-1.8.1 on RH-7.3. > > > > Thanks, > > Vadim > > > > > > > > [[alternative HTML version deleted]] > > > > ______________________________________________ > > R-help at stat.math.ethz.ch mailing list > > https://www.stat.math.ethz.ch/mailman/listinfo/r-help > > PLEASE do read the posting guide! > > http://www.R-project.org/posting-guide.html > > > > -- > The relationship between the computed price and reality is as > yet unknown. > -- From the > pac(8) manual page >
I think this is a problem with difftime (which gets called when the
subtraction in your example is invoked). The first few lines of
difftime are:
function (time1, time2, tz = "", units = c("auto",
"secs", "mins",
"hours", "days", "weeks"))
{
time1 <- as.POSIXct(time1, tz = tz)
time2 <- as.POSIXct(time2, tz = tz)
so any POSIXlt timezone gets clobbered with difftime's tz.
Vadim Ogranovich <vograno <at> evafunds.com> writes:
:
: Hi,
:
: I have two data sources. One records time in PST time zone, the other in
: GMT. I want to compute the difference between the two, but don't see
: how. Here is an example where I compute time difference between
: identical times each (meant to be) relative to its time zone.
:
: > as.POSIXlt("2000-05-10 10:15:00", "PST") -
as.POSIXlt("2000-05-10
: 10:15:00", "GMT")
: Time difference of 0 secs
:
: I was expecting to see 8hrs (which is the time difference between London
: and San-Francisco). Why is it so and what is the correct way of doing
: it?
:
: I use R-1.8.1 on RH-7.3.
:
: Thanks,
: Vadim
:
:
: [[alternative HTML version deleted]]
:
: ______________________________________________
: R-help <at> stat.math.ethz.ch mailing list
: https://www.stat.math.ethz.ch/mailman/listinfo/r-help
: PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
:
: