From http://xmlrpc-c.sourceforge.net/change_super_stable.html:
Release 1.25.26, released September 29, 2013.
Fix interpretation of dateTime.iso8601 variant with timezone
information. As broken, it would add up to a second to the correct
datetime or reject the message claiming that it has invalid syntax.
Always broken (recognition of the timezone variant was introduced in
Xmlrpc-c 1.20 (September 2009).