#include <time.h>
#include <stdbool.h>

static struct tm g_time_struct = {};

/* to the suprise of nobody, this is from stack overflow.
 *      https://stackoverflow.com/questions/21593692/convert-unix-timestamp-to-date-without-system-libs
 */
struct tm *gmtime(const time_t *timer)
{
    unsigned int seconds, minutes, hours, days, year, month;
    unsigned int day_of_week;
    seconds = *timer;

    /* calculate minutes */
    minutes  = seconds / 60;
    seconds -= minutes * 60;
    /* calculate hours */
    hours    = minutes / 60;
    minutes -= hours   * 60;
    /* calculate days */
    days     = hours   / 24;
    hours   -= days    * 24;

    /* Unix time starts in 1970 on a Thursday */
    year      = 1970;
    day_of_week = 4;

    while(1) {
        bool     leap_year   = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
        uint16_t days_in_year = leap_year ? 366 : 365;
        if (days >= days_in_year) {
            day_of_week += leap_year ? 2 : 1;
            days      -= days_in_year;
            if (day_of_week >= 7) {
                day_of_week -= 7;
            }

            ++year;
        } else {
            g_time_struct.tm_yday = days;
            day_of_week  += days;
            day_of_week  %= 7;

            /* calculate the month and day */
            static const uint8_t days_in_month[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
            for (month = 0; month < 12; ++month) {
                uint8_t dim = days_in_month[month];

                /* add a day to feburary if this is a leap year */
                if (month == 1 && leap_year) {
                    ++dim;
                }

                if (days >= dim) {
                    days -= dim;
                } else {
                    break;
                }
            }

            break;
        }
    }

    g_time_struct.tm_sec  = seconds;
    g_time_struct.tm_min  = minutes;
    g_time_struct.tm_hour = hours;
    g_time_struct.tm_mday = days + 1;
    g_time_struct.tm_mon  = month;
    g_time_struct.tm_year = year - 1900;
    g_time_struct.tm_wday = day_of_week;

    return &g_time_struct;
}