How do you convert dates/times from one time zone to another in R?

Question

If I have a date like this in London time: "2009-06-03 19:30", how can I convert it to the equivalent time in the US West Coast?

Answer 1

Package lubridate holds two functions to convert timezones. According to the help pages:

force_tz returns a date-time that has the same clock time as x in the new time zone.

force_tz(time, tzone = "America/Los_Angeles") 

with_tz changes the time zone in which an instant is displayed. The clock time displayed for the instant changes, but the moment of time described remains the same.

with_tz(time, tzone = "America/Los_Angeles") 

Answer 2

Use attr() (base R) or lubridate::with_tz() (tidyverse) to view the same moment of time in a different time zone.

The internal value of a POSIXct object is always in UTC (number of seconds since the beginning of 1970; see ?DateTimeClasses, but the displayed time when printed is determined by the tzone attribute. So change the tzone attribute (via attr() or lubridate::with_tz()) to display/print the same moment of time in a different time zone.

For example:

pb.txt <- "2009-06-03 19:30"   pb.date <- as.POSIXct(pb.txt, tz="Europe/London")  

Note tzone attribute determines time zone used when displayed/printed:

attributes(pb.date)$tzone [1] "Europe/London"  pb.date [1] "2009-06-03 19:30:00 BST" 

Note that internal value is seconds since origin in UTC, regardless of tzone attribute:

as.numeric(pb.date) [1] 1244053800 

In base R, the tzone attribute can be set/changed using attr():

attr(pb.date, "tzone") <- "America/Los_Angeles"  pb.date   [1] "2009-06-03 11:30:00 PDT"   

Equivalently, use lubridate::with_tz() (which also uses attr):

pb.date <- lubridate::with_tz(pb.date, "America/Los_Angeles") 

Note that display time is now local clock time in PDT:

pb.date   [1] "2009-06-03 11:30:00 PDT" 

but internal value has not changed:

as.numeric(pb.date) [1] 1244053800 

so tzone does not affect any operations, only the displayed value when printed.