Converts UTC offsets such as "-05:00" (5 hours behind UTC) into the
matching fixed-offset Olson time zone. Note that Etc/GMT zones use an
inverted sign convention, so "-05:00" is "Etc/GMT+5".
Examples
tzoffset_to_tz(c("+00:00", "-05:00", "+01:00"))
#> [1] "Etc/GMT0" "Etc/GMT+5" "Etc/GMT-1"