source: azure_iot_hub/trunk/musl-1.1.18/src/time/__secs_to_tm.c@ 388

#include "time_impl.h"
#include <limits.h>

/* 2000-03-01 (mod 400 year, immediately after feb29 */
#define LEAPOCH (946684800LL + 86400*(31+29))

#define DAYS_PER_400Y (365*400 + 97)
#define DAYS_PER_100Y (365*100 + 24)
#define DAYS_PER_4Y   (365*4   + 1)

int __secs_to_tm(long long t, struct tm *tm)
{
        long long days, secs, years;
        int remdays, remsecs, remyears;
        int qc_cycles, c_cycles, q_cycles;
        int months;
        int wday, yday, leap;
        static const char days_in_month[] = {31,30,31,30,31,31,30,31,30,31,31,29};

        /* Reject time_t values whose year would overflow int */
        if (t < INT_MIN * 31622400LL || t > INT_MAX * 31622400LL)
                return -1;

        secs = t - LEAPOCH;
        days = secs / 86400;
        remsecs = secs % 86400;
        if (remsecs < 0) {
                remsecs += 86400;
                days--;
        }

        wday = (3+days)%7;
        if (wday < 0) wday += 7;

        qc_cycles = days / DAYS_PER_400Y;
        remdays = days % DAYS_PER_400Y;
        if (remdays < 0) {
                remdays += DAYS_PER_400Y;
                qc_cycles--;
        }

        c_cycles = remdays / DAYS_PER_100Y;
        if (c_cycles == 4) c_cycles--;
        remdays -= c_cycles * DAYS_PER_100Y;

        q_cycles = remdays / DAYS_PER_4Y;
        if (q_cycles == 25) q_cycles--;
        remdays -= q_cycles * DAYS_PER_4Y;

        remyears = remdays / 365;
        if (remyears == 4) remyears--;
        remdays -= remyears * 365;

        leap = !remyears && (q_cycles || !c_cycles);
        yday = remdays + 31 + 28 + leap;
        if (yday >= 365+leap) yday -= 365+leap;

        years = remyears + 4*q_cycles + 100*c_cycles + 400LL*qc_cycles;

        for (months=0; days_in_month[months] <= remdays; months++)
                remdays -= days_in_month[months];

        if (months >= 10) {
                months -= 12;
                years++;
        }

        if (years+100 > INT_MAX || years+100 < INT_MIN)
                return -1;

        tm->tm_year = years + 100;
        tm->tm_mon = months + 2;
        tm->tm_mday = remdays + 1;
        tm->tm_wday = wday;
        tm->tm_yday = yday;

        tm->tm_hour = remsecs / 3600;
        tm->tm_min = remsecs / 60 % 60;
        tm->tm_sec = remsecs % 60;

        return 0;
}