How to convert from UTC to local time in C?

Question

It\'s a simple question, but the solution appears to be far from simple. I would like to know how to convert from UTC to local time. I am looking for a solution in C that\'s

Answer 1

//working stand alone function adjusting UTC to local date and time
//globals(unsigned integers): gps.Mth, gps.Yr, gps.Hm (eg:2115 for 21:15)
//adjust date and time according to UTC
//tz(timezone) eg: 1100, for 11 hours, tzdir: 1 forward, 0 backwards            





    void AdjustUTCToTimeZone(u16 tz, u8 tzdir){
    u8 maxDayInAnyMonth[13] = {0,31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; //gps.Mth 1-12 (not zero)
        if(gps.Yr%4==0){maxDayInAnyMonth[2]=29;}//adjust for leapyear
        u8 maxDayUtcMth =maxDayInAnyMonth[gps.Mth];
        u8 maxDayPrevMth=maxDayInAnyMonth[gps.Mth-1];
        if(!maxDayPrevMth){maxDayPrevMth=31;} //month before utc month

        u16 hr=(gps.Hm/100)*100;u16 m=gps.Hm-hr;  //2115 --> 2100 hr and 15 min
        if(tzdir){//adjusting forwards
            tz+=gps.Hm;
            if(tz>2400){gps.Hm=tz-2400;gps.Day++;                //spill over to next day
                  if(gps.Day>maxDayUtcMth){ gps.Day=1;gps.Mth++; //spill over to next month
                      if(gps.Mth>12){gps.Mth=1; gps.Yr++;        //spill over to next year
                      }
                  }
            }else{gps.Hm=tz;}
        }else{//adjusting backwards
            if(tz>gps.Hm){gps.Hm=(2400-(tz-hr))+m;gps.Day--;  // back to previous day
                  if(gps.Day==0){                             //back to previous month
                     gps.Mth--;gps.Day=maxDayPrevMth;
                     if(!gps.Mth){gps.Mth=12;                 //back to previous year
                        gps.Yr--;
                     }
                  }
            }else{gps.Hm-=tz;}
        }
    }