The rfc-date package is a small, portable set of functions for encoding and decoding various internet date/time formats to and from Common Lisp universal times. It requires the re package, which is also a portable, Common Lisp library.
Parsing an internet date/time string is done using one function:
(encode-universal-rfc-time string &optional format)
The format parameter should be a keyword identifying one of the supported internet formats (:rfc822, :rfc850, :rfc1036, :rfc1123, :rfc2822, :rfc3339, :atom, :cookie, :rss, :iso8601, or :w3c), and defaults to :rfc822.
Decoding a universal time into a string done with the reverse, decoding function:
(decode-universal-rfc-time universal-time &optional format)
Again, the format parameter defaults to :rfc822.
CL-USER > (decode-universal-rfc-time (get-universal-time) :rfc3339)
"2013-08-29T12:09:30-06:00"
CL-USER > (encode-universal-rfc-time * :rfc3339)
3586745370
If you'd like to test each of the formats, see how they end up printing dates, etc, you can use the (unexported) function rfc-date::unit-test. It optionally takes a universal time argument that defaults to get-universal-time. It will run through all the supported formats, convert the time to a string, then parse the string and verify that it got the same time back out.
While no helper function exists in the package for converting between date/time formats, it's easy enough to code one up...
(defun convert-rfc-time (time-string from to)
"Parse a date/time string in one format and conver it to another."
(decode-universal-rfc-time (encode-universal-rfc-time time-string from) to))