When I’m adding events to the R Community Calendar, I can only enter dates and times in my local time zone (US Pacific Time). This is problematic when I need to enter events in other countries: I have to do some fancy (and unreliable) mental gymnastics to, say, convert a future date and time in London into my local time to make sure it’s accurately listed on the calendar. So I decided to tackle the problem with R and learned a lot about dates, times, and time zones in the process.
pb.txt <- “2009-06-03 19:30”
I’ve used a standardized, unambiguous time format: year-month-date eliminates the confusion between US and British-style dates, and a 24-hour clock eliminates the AM/PM ambiguity. This format is required by the as.POSIXct function, which I’ll use to convert it to an object in R representing the date and time in the London time zone:
pb.date <- as.POSIXct(pb.txt, tz=“Europe/London”)
Now, I can convert it to my local timezone using the format function (documented in the help for strptime):
> format(pb.date, tz=“America/Los_Angeles”,usetz=TRUE)
[1] “2009-06-03 11:30:00 PDT”
d <- c(“2009-03-07 12:00”, “2009-03-08 12:00”, “2009-03-28 12:00”, “2009-03-29 12:00”, “2009-10-24 12:00”, “2009-10-25 12:00”, “2009-10-31 12:00”, “2009-11-01 12:00”)
t1 <- as.POSIXct(d,“America/Los_Angeles”)
cbind(US=format(t1),UK=format(t1,tz=“Europe/London”))
- You can work with dates represented as character strings in various formats like “12/25/2008” or “8 Jan 2009 2:43 PM” using the function strptime
- It’s a bit tricky to find valid time zones to use with the tz= argument. Common timezone abbreviations like “BST” (British Summer Time) sometimes work, but might not mean what you think they mean: EST refers to a time-zone in Canada that does not observe daylight savings time, NOT US Eastern Standard Time. It’s safest to use the time zone names listed on this Wikipedia page (especially if you’re dealing with data from the archives of Louisville, Kentucky). The list of known timezones is system-specific, but these ones should work on all R implementations.
- Important warning: If you misspell a time zone, or use one that isn’t known, you don’t get an error: UTC (basically Greenwich Mean Time) is used instead. If you’re not sure, print your POSIXct object: if it looks like this: “2009-06-03 19:30:00 UTC” your time zone wasn’t recognized. On my machine this happens if I try to use “PDT” or “PST” as a time zone, even though dates are printed with those characters representing the time zone. “US/Pacific works”, but I can’t find any documentation listing time zones beginning with “US/”.
- You might think you can convert a POSIXct object from one time zone to another like this: as.POSIXct(pb.date,tz=”US/Pacific”), but it doesn’t work. You can strip the time-zone information from the object first with c: as.POSIXct(c(pb.date),tz=”US/Pacific”). But it’s probably safest just to use format.
- POSIXlt objects represent times like POSIXct objects do, but you only want to use them if you need to extract information like the day of the week, or if daylight savings is in effect. I had difficulties working with time zones using POSIXlt objects, so you’ll probably want to stick with as.POSIXct for most purposes. The differences between the two are documented in ?DateTimeClasses.
