undefined reference to boost::gregorian::greg_month::as_short_string() const

Boost date_time is not a header-only library. Please build the library and then add it. Simple in gcc:

gcc myapp.cpp -omyapp -lboost_date_time

(Be careful! This library sneakily appears to work as a header-only library at optimization levels -O2 and higher, due to inlining; but it will fail to link when you use lower optimization levels where the compiler's inliner isn't as aggressive.)