Converting epoch time to "real" date/time

Question

What I want to do is convert an epoch time (seconds since midnight 1/1/1970) to "real" time (m/d/y h:m:s)

So far, I have the following algorithm, which to me feels a bit ugly:

void DateTime::splitTicks(time_t time) {     seconds = time % 60;     time /= 60;     minutes = time % 60;     time /= 60;     hours = time % 24;     time /= 24;      year = DateTime::reduceDaysToYear(time);     month = DateTime::reduceDaysToMonths(time,year);     day = int(time); }  int DateTime::reduceDaysToYear(time_t &days) {     int year;     for (year=1970;days>daysInYear(year);year++) {         days -= daysInYear(year);     }     return year; }  int DateTime::reduceDaysToMonths(time_t &days,int year) {     int month;     for (month=0;days>daysInMonth(month,year);month++)         days -= daysInMonth(month,year);     return month; } 

you can assume that the members seconds, minutes, hours, month, day, and year all exist.

Using the for loops to modify the original time feels a little off, and I was wondering if there is a "better" solution to this.

Answer 1

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; } 

Answer 2

The standard library provides functions for doing this. gmtime() or localtime() will convert a time_t (seconds since the epoch, i.e.- Jan 1 1970 00:00:00) into a struct tm. strftime() can then be used to convert a struct tm into a string (char*) based on the format you specify.

see: http://www.cplusplus.com/reference/clibrary/ctime/

Date/time calculations can get tricky. You are much better off using an existing solution rather than trying to roll your own, unless you have a really good reason.