> std::string nowString = boost::posix_time::to_iso_extended_string( now );

Is there any way to change the 'T' char separator with another character or string?

std::string nowString = boost::posix_time::to_simple_string ( now );

http://www.boost.org/doc/libs/1_43_0/doc/html/date_time/posix_time.html#date_time.posix_time.ptime_class 
(conversion to string)