greg_month.hpp includes files in the following order: #include "boost/date_time/constrained_value.hpp" #include "boost/date_time/date_defs.hpp" #include "boost/shared_ptr.hpp" #include "boost/date_time/compiler_config.hpp" Should not compiler_config.hpp come first? As it is the date_time library fails to compile on Interix (sigh) as <time.h> is included before boost/config/user.hpp is sourced with a few function signatures omitted as a result. Best, V.