TempleOS-Rijn/LT/OSMain/DATE.CPZ

I4 YearStartDate(I4 year)
{
  I4 y1=year-1;
  I4 yd4000=y1/4000,yd400=y1/400,yd100=y1/100,yd4=y1/4;
  return year*365+yd4-yd100+yd400-yd4000;
}

LTDate ToLTDate(LTDateStruct *d)
{
  LTDate ltdt;
  I4 i1,i2;
  i1=YearStartDate(d->year);
  i2=YearStartDate(d->year+1);
  if (i2-i1==365)
     i1+=month_start_days1[d->month-1];
   else
     i1+=month_start_days2[d->month-1];
  ltdt.date=i1+d->day_of_month-1;
  ltdt.time=DivU8((d->ten_thousandths+10000*(d->seconds+60*(d->minutes+60*d->hours)))<<21,(15*15*3*625));
  return ltdt;
}

U1 DayOfWeek(I4 i)
{
  return (i+LT_BASE_DAY_OF_WEEK) % 7;
}

void ToLTDateStruct(LTDateStruct *d,LTDate ltdt)
{
  I4 i,date=ltdt.date;
  U4 k;
  d->day_of_week=DayOfWeek(date);
  d->year=DivI8(MulI8(date,100000),36524225);
  i=YearStartDate(d->year);
  date-=i;
  if (YearStartDate(d->year+1)-i==365) {
    k=0;
    while (date>=month_start_days1[k+1] && k<11)
      k++;
    date-=month_start_days1[k];
  } else {
    k=0;
    while (date>=month_start_days2[k+1] && k<11)
      k++;
    date-=month_start_days2[k];
  }
  d->month=k+1;
  d->day_of_month=date+1;
  k=MulU8(625*15*15*3,ltdt.time)>>21+1;
  d->ten_thousandths=k%10000; k/=100;
  d->hundredths=k%100;	k/=100;
  d->seconds=k%60;k/=60;
  d->minutes=k%60;
  d->hours  =k/60;
}

I4 FirstDayOfMonth(I4 i)
{
  LTDateStruct d;
  LTDate ltdt=0;
  ltdt.date=i;
  ToLTDateStruct(&d,ltdt);
  d.day_of_month=1;
  ltdt=ToLTDate(&d);
  return ltdt.date;
}

I4 LastDayOfMonth(I4 i)
{
  LTDateStruct d;
  LTDate ltdt=0;
  ltdt.date=i;
  ToLTDateStruct(&d,ltdt);
  d.month++;
  if (d.month==13) {
    d.month=0;
    d.year++;
  }
  d.day_of_month=1;
  ltdt=ToLTDate(&d);
  return ltdt.date-1;
}

I4 FirstDayOfYear(I4 i)
{
  LTDateStruct d;
  LTDate ltdt=0;
  ltdt.date=i;
  ToLTDateStruct(&d,ltdt);
  d.day_of_month=1;
  d.month=1;
  ltdt=ToLTDate(&d);
  return ltdt.date;
}

I4 LastDayOfYear(I4 i)
{
  LTDateStruct d;
  LTDate ltdt=0;
  ltdt.date=i;
  ToLTDateStruct(&d,ltdt);
  d.day_of_month=1;
  d.month=1;
  d.year++;
  ltdt=ToLTDate(&d);
  return ltdt.date-1;
}


I8 Bcd2Bin(U8 b)
{
  I8 i,result=0;
  for (i=0;i<16;i++) {
    result=MulU8(result,10)+DivU8(b,0x1000000000000000);
    b<<=4;
  }
  return result;
}


void GetCurTimeLTDateStruct(LTDateStruct *r)
{
  I8 i;
  U8 old_flags=GetFlags;
  U1 *p=r;

  MemSet(r,0,sizeof(LTDateStruct));
  Cli;
  do OutP(0x70,0x0A);
  while (InP(0x71) & 0x80);

  OutP(0x70,0);
  p[3]=InP(0x71);
  OutP(0x70,2);
  p[4]=InP(0x71);
  OutP(0x70,4);
  p[5]=InP(0x71);

  OutP(0x70,6);
  p[6]=InP(0x71);
  OutP(0x70,7);
  p[7]=InP(0x71);
  OutP(0x70,8);
  p[8]=InP(0x71);
  OutP(0x70,9);
  p[9]=InP(0x71);

  SetFlags(old_flags);

  OutP(0x70,0x0B);
  if (!(InP(0x71) & 4)) {
    for (i=3;i<10;i++)
      p[i]=Bcd2Bin(p[i]);
  }
  r->year+=2000;
}

LTDate GetCurTimeLTDate()
{
  LTDateStruct r;
  GetCurTimeLTDateStruct(&r);
  return ToLTDate(&r);
}

void LTDateToISO(ISODate *dst,LTDate ltdate)
{
  LTDateStruct d;
  ToLTDateStruct(&d,ltdate);
  dst->year=d.year-ISO_BASE_YEAR;
  dst->mon=d.month;
  dst->day=d.day_of_month;
  dst->hour=d.hours;
  dst->min=d.minutes;
  dst->sec=d.seconds;
  dst->hund=d.hundredths;
}

LTDate ISOToLTDate(ISODate *dt)
{
  LTDateStruct ltdt;
  ltdt.day_of_month=dt->day;
  ltdt.month=dt->mon;
  ltdt.year=dt->year+ISO_BASE_YEAR;
  ltdt.ten_thousandths=dt->hund*100;
  ltdt.hundredths=dt->hund;
  ltdt.seconds=dt->sec;
  ltdt.minutes=dt->min;
  ltdt.hours=dt->hour;
  return ToLTDate(&ltdt);
}