This is a nightmare to implement (unless you go library by library and do it by hand, or you have significant compiler help), as e.g. functions may not directly

This is a nightmare to implement (unless you go library by library and do it by hand, or you have significant compiler help), as e.g. functions may not directly use a struct with time_t but indirectly assume something about the size of the struct. Note that for example the std::string transition did not do this.