This sounds simple although I can't find a quick answer at the BCGDATE website. It sounds like you need to #include a .h file from BCGDATE with your function defined. Check to make sure you have the correct case, spelling, and spacing.
In addition to including a header (.h) file from BCGDATETIME, you may also need to add a library (.lib) file to your project. This library file will resolve any linker errors that you may run into when building the application.