test converters for days before 1582 (gergorian calendar)

before_greg_test.cpp

#include <cstdio>
#include <chrono>
#include <cinttypes>
#include <ctime>

struct YMD
{
    int y{};
    unsigned m{};
    unsigned d{};
};

using days = std::chrono::duration
    <int, std::ratio_multiply<std::ratio<24>, std::chrono::hours::period>::type>;

YMD civi_from_days(days dd) noexcept
{
    auto z = dd.count() + 719468;
    const int era = (z >= 0 ? z : z - 146096) / 146097;
    const unsigned doe = static_cast<unsigned>(z - era * 146097);          // [0, 146096]
    const unsigned yoe = (doe - doe/1460 + doe/36524 - doe/146096) / 365;  // [0, 399]
    const int y = static_cast<int>(yoe) + era * 400;
    const unsigned doy = doe - (365*yoe + yoe/4 - yoe/100);                // [0, 365]
    const unsigned mp = (5*doy + 2)/153;                                   // [0, 11]
    const unsigned d = doy - (153*mp+2)/5 + 1;                             // [1, 31]
    const unsigned m = mp + (mp < 10 ? 3 : -9);                            // [1, 12]
    return { y + static_cast<int>(m <= 2), m, d};
}


int main()
{
    auto input_sec = std::chrono::seconds(-61999999999);
    
    // MIDNIGHT NORMALIZATION is very risky and didn't found any documentations about it
    // probably a good discussion is here https://stackoverflow.com/a/23978037
    //const std::time_t t = input_sec.count();
    //auto* utcTime = std::gmtime(&t);
    //utcTime->tm_sec = utcTime->tm_min = utcTime->tm_hour = 0;
    //input_sec = std::chrono::seconds(std::mktime(utcTime));
    
    // I think to safe conversion date before 1582 we have to create our own calendar in ICU
    // Tests for other systems:
    // linux: "Tue Apr 19 09:21:41 LMT 0005"
    // Javascript "Tue Apr 19 0005 09:21:41 GMT-0025"
    // Python same
    // Postgres : "0005-04-17T09:46:41Z"
    // MySQL: just return NULL
    // MSSQL: crash sometimes
    const auto ymd = civi_from_days(std::chrono::duration_cast<days>(input_sec));
    printf("%d / %d / %d for %" PRId64 " seconds", ymd.y, ymd.m, ymd.d, input_sec.count());
}