[boost] [serialization][date_time] unresolved symbol 'special_value_from_string'

Hi, I'm trying to deserialize objects from an xml archive containing some gregorian dates. Using VC7.1 I get the linker error: serialization.obj : error LNK2019: unresolved external symbol "enum boost::date_time::special_values __cdecl boost::gregorian::special_value_from_string(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (?special_value_from_string@gregorian@boost@@YA?AW4special_values@date_time@2@AB V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function __catch$??$load@Vxml_wiarchive@archive@boost@@@serialization@boost@@YAXAAVxml_wi archive@archive@1@AAVdate@gregorian@1@I@Z$0 Any ideas what's causing this? Jonathan