RFC3339-timestamp is relaxed implementation of RFC3339 timestamps
using cl-yacc for parsing and a very simple timestamp class for
keeping track of fractions of a second and offset if you need it to
survive a decode/encode round trip.

A known limitation is that fractions of a second are limited to 10
digits. The parser implementation is quite relaxed about what it
passes. For instance a single space is allowed in place of a T for
date time seperation. Z or time offset is not needed at all and when
left out the parser will assume the timestamp is UTC+00:00. Fractions
are of course optional although that is also true per the RFC.

Another limitation is that dates prior to 1900-01-01 01:00:00 cannot
be represented because that is when universal time which is used
internally starts.

If you don't care for the relaxed mode of the parser you can make it
stricter by pushing :rfc3339-strict-parser to your *features* and
recompile. This will make the parser only accept true RFC3339

The other kind of timestamp that will be parsed if
:rfc3339-strict-parser is not in *features* is the specific format
used by xml-rpc. Which to my understanding is YYYYMMDDTHH:MM:SS.