[boost] boost::posix_time::ptime constructor from special values? ( eg, not_a_date_time )

Hello. Thanks for all your work in the boost::date_time code. It's very useful. I've got a question about constructing a ptime from special values. The documentation says it works, but I can't find the appropriate constructor in ptime.hpp. (I'm using 1.31.0.) Is this a bug, or am I missing something? Richard