MCVE([online-compiler](https://ideone.com/kxBgQN)):

    #include <iostream>
    #include <iomanip>
    #include <cstdlib>
    #include <sstream>
    #include <chrono>
    
    int main()
    {
        std::tm date{};
        std::istringstream ss("2021-05-1");
        ss >> std::get_time(&date, "%Y-%m-%d");
        std::cout << std::boolalpha << ss.fail() << '\n';
    }
    
Output:

> true

On VS compiler output is:

> false

[Cppreference](https://en.cppreference.com/w/cpp/io/manip/get_time) is saying that leading zeros are not requiring, is it bug in gcc or something else?