with_tz returns a date-time as it would appear in a different time zone.
The actual moment of time measured does not change, just the time zone it is
measured in. with_tz defaults to the Universal Coordinated time zone (UTC)
when an unrecognized time zone is inputted. See
for more information on how R recognizes time zones.
with_tz(time, tzone = "")
a POSIXct, POSIXlt, Date, chron date-time object or a data.frame
object. When a data.frame all POSIXt elements of a data.frame are processed
a character string containing the time zone to convert to. R must recognize the name contained in the string as a time zone on your system.
a POSIXct object in the updated time zone
#>  "2009-08-07 04:00:01 GMT"