Time zones are stored as character strings in an attribute of date-time objects. tz returns a date's time zone attribute. When used as a settor, it changes the time zone attribute. R does not come with a predefined list zone names, but relies on the user's OS to interpret time zone names. As a result, some names will be recognized on some computers but not others. Most computers, however, will recognize names in the timezone data base originally compiled by Arthur Olson. These names normally take the form "Country/City." A convenient listing of these timezones can be found at http://en.wikipedia.org/wiki/List_of_tz_database_time_zones.
tz(x) tz(x) <- value
a date-time object of class a POSIXct, POSIXlt, Date, chron, yearmon, yearqtr, zoo, zooreg, timeDate, xts, its, ti, jul, timeSeries, fts or anything else that can be coerced to POSIXlt with as.POSIXlt
timezone value to be assigned to
the first element of x's tzone attribute vector as a character string. If no tzone attribute exists, tz returns "GMT".
Setting tz does not update a date-time to display the same moment as measured
at a different time zone. See
with_tz(). Setting a new time zone
creates a new date-time. The numerical value of the hours element stays the
same, only the time zone attribute is replaced. This creates a new date-time
that occurs an integer value of hours before or after the original date-time.
If x is of a class that displays all date-times in the GMT timezone, such as chron, then R will update the number in the hours element to display the new date-time in the GMT timezone.
x <- ymd("2012-03-26") tz(x)#>  "UTC"tz(x) <- "GMT" x#>  "2012-03-26"