(datetime-format 'atom) ;=> "2015-01-12T02:01:11+09:00"
(datetime-format 'atom (current-time)) ;=> "2015-01-12T02:01:11+09:00"
(datetime-format 'atom 0) ;=> "1970-01-01T09:00:00+09:00"
(datetime-format 'atom 0 :timezone "UTC") ;=> "1970-01-01T00:00:00+00:00"
(datetime-format 'atom "2015-01-12 02:01:11") ;=> "2015-01-12T02:01:11+09:00"
(datetime-format 'atom "2015-01-12 02:01:11" :timezone "Europe/Moscow") ;=> "2015-01-12T01:01:11+03:00"
(datetime-format 'atom-utc "2015-01-12 02:01:11") ;=> "2015-01-11T17:01:11Z"
(datetime-format 'atom nil :timezone "America/New_York") ;=> "2016-05-18T13:05:41-04:00"