a utility to compare date diff and date advance^_^

#include <cassert>

typedef struct date_s date_t;

struct date_s
{
  int serial;
};

#define is_leap(y) (((y) % 400 == 0)||((0 == (y)%4) && (0 != (y) % 100)))

int year_days[2] = {365, 366};
unsigned char month_days[2][12] = {
  {
    31, //Jan
    28, //Feb
    31, //Mar
    30, //Apr
    31, //May
    30, //Jun
    31, //Jul
    31,  //Aug
    30,  //Sep
    31,  //Oct
    30,  //Nov
    31   //Dec
  },

  {
    31, //Jan
    29, //Feb
    31, //Mar
    30, //Apr
    31, //May
    30, //Jun
    31, //Jul
    31,  //Aug
    30,  //Sep
    31,  //Oct
    30,  //Nov
    31   //Dec
  }
};



enum 
{
  base_year =2013, //the base date shall be Jan, 1
  base_weekday = 2 //0: Sunday, 1: Monday ... 6: Saturday
};


date_t to_date(int yyyymmdd)
{
  int y, m, d, serial = 0;
  y = yyyymmdd / 10000;
  m = yyyymmdd / 100 % 100;
  d = yyyymmdd % 100;

  serial += d - 1;

  while(m > 1)
  {
    --m;
    serial += month_days[is_leap(y)][m-1];
  } 

  while(y > base_year)
  {
    serial += year_days[is_leap(y)];
    --y;
  }

  while(y < base_year)
  {
    serial -=  year_days[is_leap(y)];
    ++y;
  }
  
  date_t date = {serial};
  return date;
}

int date_diff(date_t const& d1, date_t const& d2)
{
  return d1.serial - d2.serial;
}

date_t date_advance(date_t const& d, int diff)
{
  date_t date = {d.serial + diff};
  return date;
}


//only for human readable output/log
int to_yyymmdd(date_t const& date)
{
  int y = base_year, m = 1, d = 1;
  while(to_date(y*10000+ m * 100 + d).serial < date.serial)
  {
    ++d;
    if(d > month_days[is_leap(y)][m-1])
    {
      d = 1;
      ++m;
    }
    if(m  > 12)
    {
      m = 1;
      ++y;
    }
  }
  while(to_date(y*10000+ m * 100 + d).serial > date.serial)
  {
    
    --d;
    if(0 == d)
    {
      --m;
      if(m == 0)
      {
        m = 12;
        --y;
      }
      d = month_days[is_leap(y)][m-1];
    }   
  }
  return y*10000+100*m+d;
}
//only for human readable output/log
int weekday(date_t const&  date)
{
  int remaining = date.serial % 7;
  int w = base_weekday;
  while(remaining > 0)
  {
    ++w;
    if(w == 7)
    {
      w = 0;
    }
    --remaining;
  }

  while(remaining < 0)
  {
    --w;
    if(w == -1)
    {
      w = 6;
    }
    ++remaining;
  }
  return w;
}
char const* const weekdays[7] = {"Sunday", "Monday", "Tuesday", 
  "Wendsday", "Thursday", "Friday", "Staturday"};

int main()
{
  date_t d1 = to_date(20120304), d2 = to_date(20140418);
  int diff = date_diff(d1, d2);//d1-d2 == diff
  date_t d3 = date_advance(d1, -diff); //d2 = d1 - diff 
  assert(d3.serial == d2.serial);
  return 0;
}