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 timestamps. 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.