*BNF

;; RFC 2424, draft-ietf-vpim-vpimv2r2-dur-03 を元に作成。

 Content-Duration     = "Content-Duration:" SP duration [CFWS]
                      / obs-Content-Duration
 obs-Content-Duration = "Content-Duration" [FWS] ":" [CFWS] duration [CFWS]
 duration             = 1*10DIGIT	;; 1〜2147483647 (= 2^31-1)

*See also
-<urn:ietf:rfc:2424>
-<urn:ietf:id:draft-ietf-vpim-vpimv2r2-dur-03>
-[[MIME]]
-[[RFC822と仲間達の頭領域名]]