How to convert std::chrono::time_point to std::tm without using time_t?

Question

I would like to print or extract year/month/day values.

I don\'t want to use time_t because of the year 2038 problem, but all examples I found on the In

Answer 1

I have used Howard Hinnant's date library to write a function that converts from time_point to struct tm:

template 
std::tm to_calendar_time(std::chrono::time_point tp)
{
    using namespace date;
    auto date = floor(tp);
    auto ymd = year_month_day(date);
    auto weekday = year_month_weekday(date).weekday_indexed().weekday();
    auto tod = make_time(tp - date);
    days daysSinceJan1 = date - sys_days(ymd.year()/1/1);

    std::tm result;
    std::memset(&result, 0, sizeof(result));
    result.tm_sec   = tod.seconds().count();
    result.tm_min   = tod.minutes().count();
    result.tm_hour  = tod.hours().count();
    result.tm_mday  = unsigned(ymd.day());
    result.tm_mon   = unsigned(ymd.month()) - 1u; // Zero-based!
    result.tm_year  = int(ymd.year()) - 1900;
    result.tm_wday  = unsigned(weekday);
    result.tm_yday  = daysSinceJan1.count();
    result.tm_isdst = -1; // Information not available
    return result;
}

This effectively bypasses time_t with its lurking Y2038 problem on 32-bit systems. This function has been contributed to this GitHub wiki, where I hope others will contribute other useful examples and recipes.