The Core Parts

int main(int argc, const char** argv) {
    if (argc <= 1)
        return 1;
    try {
        const auto paths = CollectPaths(argv[1]);
        if (paths.empty()) {
            std::cout << "No files to process...\n";
            return 0;
        }
        const auto startDate = argc > 2 ? Date(argv[2]) : Date();
        const auto endDate = argc > 3 ? Date(argv[3]) : Date();
        const auto results = CalcResults(paths, startDate, endDate);
        ShowResults(startDate, endDate, results);
    }
    catch (const std::filesystem::filesystem_error& err) {
        std::cerr << "filesystem error! " << err.what() << '\n';
    }
    catch (const std::runtime_error& err) {
        std::cerr << "runtime  error! " << err.what() << '\n';
    }
    return 0;
}