[boost] BOOST_DATE_TIME_NO_LIB

Without the #define, boost tries to automatically link with the appropriate library. It just so happens that my boost installation does not have this library. So I was wondering if I even need this library? Aren't all boost classes compiled via hpp. I #defined BOOST_DATE_TIME_NO_LIB in my header file and the program compiles and works fine. So what is the purpose of this definition. I am using boost 1.32 along with Visual Studio 6.0. I have compiled it on Linux successfully, but I have not ran it yet.