Converting epoch time to “real” date/time

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Be careful about leap years in your daysInMonth function.

If you want very high performance, you can precompute the pair to get to month+year in one step, and then calculate the day/hour/min/sec.

A good solution is the one in the gmtime source code:

/*
 * gmtime - convert the calendar time into broken down time
 */
/* $Header: gmtime.c,v 1.4 91/04/22 13:20:27 ceriel Exp $ */

#include        <time.h>
#include        <limits.h>
#include        "loc_time.h"

struct tm *
gmtime(register const time_t *timer)
{
        static struct tm br_time;
        register struct tm *timep = &br_time;
        time_t time = *timer;
        register unsigned long dayclock, dayno;
        int year = EPOCH_YR;

        dayclock = (unsigned long)time % SECS_DAY;
        dayno = (unsigned long)time / SECS_DAY;

        timep->tm_sec = dayclock % 60;
        timep->tm_min = (dayclock % 3600) / 60;
        timep->tm_hour = dayclock / 3600;
        timep->tm_wday = (dayno + 4) % 7;       /* day 0 was a thursday */
        while (dayno >= YEARSIZE(year)) {
                dayno -= YEARSIZE(year);
                year++;
        }
        timep->tm_year = year - YEAR0;
        timep->tm_yday = dayno;
        timep->tm_mon = 0;
        while (dayno >= _ytab[LEAPYEAR(year)][timep->tm_mon]) {
                dayno -= _ytab[LEAPYEAR(year)][timep->tm_mon];
                timep->tm_mon++;
        }
        timep->tm_mday = dayno + 1;
        timep->tm_isdst = 0;

        return timep;
}

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