GPSTime To UTC Date Time

const static double gpst0[]={1980,1, 6,0,0,0};
typedef struct {        /* time struct */
    time_t time;        /* time (s) expressed by standard time_t */
    double sec;         /* fraction of second under 1 s */
} gtime_t;
extern gtime_t epoch2time(const double *ep)
{
    const int doy[]={1,32,60,91,121,152,182,213,244,274,305,335};
    gtime_t time={0};
    int days,sec,year=(int)ep[0],mon=(int)ep[1],day=(int)ep[2];
    
    if (year<1970||2099<year||mon<1||12<mon) return time;
    
    /* leap year if year%4==0 in 1901-2099 */
    days=(year-1970)*365+(year-1969)/4+doy[mon-1]+day-2+(year%4==0&&mon>=3?1:0);
    sec=(int)floor(ep[5]);
    time.time=(time_t)days*86400+(int)ep[3]*3600+(int)ep[4]*60+sec;
    time.sec=ep[5]-sec;
    return time;
}
extern gtime_t gpst2time(int week, double sec)
{
    gtime_t t=epoch2time(gpst0);
    
    if (sec<-1E9||1E9<sec) sec=0.0;
    t.time+=86400*7*week+(int)sec;
    t.sec=sec-(int)sec;
    return t;
}
extern gtime_t timeadd(gtime_t t, double sec)
{
    double tt;
    
    t.sec+=sec; tt=floor(t.sec); t.time+=(int)tt; t.sec-=tt;
    return t;
}
extern gtime_t GPSTime2UTCTime(int week,double sec,double leapsec)
{
    gtime_t gpst = gpst2time(week,sec);
    return timeadd(gpst,-leapsec);
}