The new `datetime_format` key will offer the possibility of
providing a different date/time format than the default one
(`'%A %d %B %Y %H:%M:%S %z'`).
- The `iso8601` value can be used as an `'%Y-%m-%dT%H:%M:%S.%f'`
alias (format of the ISO 8601 date/time standard).
The code has changed from using the `time` API to the `datetime`
one in order to support sub-second precision (needed by the ISO
8601 format, for example).
Signed-off-by: Pablo Mendez Hernandez <pablomh@redhat.com>
