I get confused by this also, but I believe your time zone is US/Pacific, which specifies both the offset from UTC and the dates on which we switch between 'standard' (winter) and 'daylight savings' (summer). I think you would have to create a new time zone entry that is always UTC+8 hours, or whatever, if you want to use standard time at all times.
I usually lie and use tz="UTC" when using data in local standard time (e.g., tide tables in the US). Bill Dunlap TIBCO Software wdunlap tibco.com On Fri, Oct 30, 2015 at 12:30 PM, Clint Bowman <cl...@ecy.wa.gov> wrote: > Bill, > > Your final words, "changes in spring and fall" reminds me of a problem I > have yet to solve. Most of my data is logged in standard time (no daylight > times) but often I see the note "daylight time encountered switching to > UTC" even when I've specified "tz="PST". > > I hope I've been missing something simple--any suggestions? > > TIA > > Clint > > Clint Bowman INTERNET: cl...@ecy.wa.gov > Air Quality Modeler INTERNET: cl...@math.utah.edu > Department of Ecology VOICE: (360) 407-6815 > PO Box 47600 FAX: (360) 407-7534 > Olympia, WA 98504-7600 > > USPS: PO Box 47600, Olympia, WA 98504-7600 > Parcels: 300 Desmond Drive, Lacey, WA 98503-1274 > > On Fri, 30 Oct 2015, William Dunlap wrote: > > You can use difftime objects to get the amount of time since the start of >> the current day. E.g., >> > dateTime <- as.POSIXlt( c("2015-10-29 00:50:00", >> + "2015-10-29 09:30:00", "2015-10-29 21:10:00", "2015-10-30 00:50:00", >> + "2015-10-30 09:30:00", "2015-10-30 21:10:00", "2015-10-31 00:50:00", >> + "2015-10-31 10:30:00")) >> > date <- trunc(dateTime, units="days") >> > sinceMidnight <- difftime(dateTime, date, units="mins") >> > sinceMidnight >> Time differences in mins >> [1] 50 570 1270 50 570 1270 50 630 >> >> I use difftime(x, y, units=) instead of the similar x-y because the latter >> chooses >> the units based on how far apart x and y are, while the former gives me >> consistent >> units: >> > dateTime[1] - date[1] >> Time difference of 50 mins >> > as.numeric(.Last.value) >> [1] 50 >> > dateTime[5:6] - date[5:6] >> Time differences in hours >> [1] 9.50000 21.16667 >> > as.numeric(.Last.value) >> [1] 9.50000 21.16667 >> >> Depending on what you are using this for, you might want to compute time >> since 3am >> of the current day so you don't get discontinuities for most times when >> the >> time >> changes in spring and fall. >> >> >> >> Bill Dunlap >> TIBCO Software >> wdunlap tibco.com >> >> On Fri, Oct 30, 2015 at 10:35 AM, Daniel Nordlund < >> djnordl...@frontier.com> >> wrote: >> >> I have a data frame with date/times represented as charaacter strings and >>> and a value at that date/time. I want to get the mean value for each >>> time >>> of day, across days, and then plot time of day on the x-axis and means on >>> the y-axis. R doesn't appear to have a built-in time of day time type >>> (independent of a date), unless I have missed something. What is the best >>> way to create a time variable so that I can aggregate and plot by time of >>> day, with time labelled in HH:MM format. My current approach is to >>> convert >>> all date/times to the same date. I can then manage the rest of what I >>> want >>> with ggplot2. But I am wondering if there is an easier/better way to do >>> deal with time of day. >>> >>> Here is a sample data frame. >>> >>> df <- structure(list(date = structure(1:8, .Label = c("2015-10-29 >>> 00:50:00", >>> "2015-10-29 09:30:00", "2015-10-29 21:10:00", "2015-10-30 00:50:00", >>> "2015-10-30 09:30:00", "2015-10-30 21:10:00", "2015-10-31 00:50:00", >>> "2015-10-31 10:30:00"), class = "factor"), value = c(88L, 17L, >>> 80L, 28L, 23L, 39L, 82L, 79L)), .Names = c("date", "value"), row.names = >>> c(NA, >>> -8L), class = "data.frame") >>> >>> >>> Any suggestions appreciated. >>> >>> Dan >>> >>> -- >>> Daniel Nordlund >>> Bothell, WA USA >>> >>> ______________________________________________ >>> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >>> 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. >>> >>> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> 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. >> >> [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.