(if stream
(write-string string stream)
string)))
+ (:iso8601
+ (format stream "~4,'0D-~2,'0D-~2,'0DT~2,'0D:~2,'0D:~2,'0D.~DZ"
+ year month day hour minute second usec))
(t
(format stream "~2,'0D~A~2,'0D~A~2,'0D~A~2,'0D~A~2,'0D~A~2,'0D.~6,'0D"
year date-separator month date-separator day