seconds to tm

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>

typedef uint32_t _u32;

typedef struct
{
    /* time */
    _u32                sl_tm_sec;
    _u32                sl_tm_min;
    _u32                sl_tm_hour;
    /* date */
    _u32                sl_tm_day; /* 1-31 */
    _u32                sl_tm_mon; /* 1-12 */
    _u32                sl_tm_year; /*  YYYY 4 digits  */
    _u32                sl_tm_week_day; /* not required */
    _u32                sl_tm_year_day; /* not required */
    _u32                reserved[3];
} SlDateTime_t;

#define DAY 86400
#define YEAR    (DAY * 365)
#define YEAR4   (YEAR * 4 + DAY)

static int secs2tm(time_t t, SlDateTime_t *st)
{
    /* days in a month, starting March */
    uint8_t dom[] = { 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 31, 29 };
    int i;

    /* start just after 29 Feb 1972, the first leap day in the UNIX history*/
    st->sl_tm_year = 1972;
    t -= 68256000;

    if (t > YEAR4) {
        st->sl_tm_year += t / YEAR4 * 4;
        t %= YEAR4;
    }
    if (t > YEAR) {
        st->sl_tm_year += t / YEAR;
        t %= YEAR;
        dom[11]--;
    }
    for (i = 0; i < 11; i++) {
        if(t < DAY * dom[i])
            break;
        t -= DAY * dom[i];
    }
    st->sl_tm_mon = i + 3;
    if (st->sl_tm_mon > 12) {
        st->sl_tm_year++;
        st->sl_tm_mon -= 12;
    }

    st->sl_tm_day = t / DAY + 1;
    t %= DAY;

    st->sl_tm_hour = t / 3600;
    t %= 3600;

    st->sl_tm_min = t / 60;
    t %= 60;

    st->sl_tm_sec = t;

    return 0;
}

/*
  test with:

while : ; do
    d=$RANDOM$RANDOM
    [ $d -lt 68256000 ] && continue
    [ "$(./s2t $d)" = "$(date -u -d"@$d" +"%F %T")" ] || echo "error @$d: $(./s2t $d) != ($(date -u -d"@$d" +"%F %T"))"
done

 */

int main(int argc, char *argv[])
{
    SlDateTime_t tm;
    uint32_t date;

    if(argc < 2) {
        fprintf(stderr, "usage: %s <secs from epoch>\n", *argv);
        return 1;
    }
    date = atoi(argv[1]);

    secs2tm(date, &tm);

    printf("%04u-%02u-%02u %02u:%02u:%02u\n",
        tm.sl_tm_year,
        tm.sl_tm_mon,
        tm.sl_tm_day,
        tm.sl_tm_hour,
        tm.sl_tm_min,
        tm.sl_tm_sec
    );

    return 0;
}