Formatting: date-time — posixt-formatting
This is a POSIXct method for the date_format() generic.
date_format() formats a date-time (POSIXct) using a format string.
If format is NULL, a default format of "%Y-%m-%dT%H:%M:%S%Ez[%Z]" is
used. This matches the default format that date_time_parse_complete()
parses. Additionally, this format matches the de-facto standard extension to
RFC 3339 for creating completely unambiguous date-times.