#include <stdio.h> 
#include <stdlib.h> 
#include <time.h> 


int GetWeek( struct tm* date)
{
	if (NULL == date)
	{
		return 0; // or -1 or throw exception
	}
	if (::mktime(date) < 0) // Make sure _USE_32BIT_TIME_T is NOT defined.
	{
		return 0; // or -1 or throw exception
	}
	// The basic calculation:
	// {Day of Year (1 to 366) + 10 - Day of Week (Mon = 1 to Sun = 7)} / 7
	int monToSun = (date->tm_wday == 0) ? 7 : date->tm_wday; // Adjust zero indexed week day
	int week = ((date->tm_yday + 11 - monToSun) / 7); // Add 11 because yday is 0 to 365.
 
	// Now deal with special cases:
	// A) If calculated week is zero, then it is part of the last week of the previous year.
	if (week == 0)
	{
		// We need to find out if there are 53 weeks in previous year.
		// Unfortunately to do so we have to call mktime again to get the information we require.
		// Here we can use a slight cheat - reuse this function!
		// (This won't end up in a loop, because there's no way week will be zero again with these values).
		tm lastDay = { 0 };
		lastDay.tm_mday = 31;
		lastDay.tm_mon = 11;
		lastDay.tm_year = date->tm_year - 1;
		// We set time to sometime during the day (midday seems to make sense)
		// so that we don't get problems with daylight saving time.
		lastDay.tm_hour = 12;
		week = GetWeek(&lastDay);
	}
	// B) If calculated week is 53, then we need to determine if there really are 53 weeks in current year
	//    or if this is actually week one of the next year.
	else if (week == 53)
	{
		// We need to find out if there really are 53 weeks in this year,
		// There must be 53 weeks in the year if:
		// a) it ends on Thurs (year also starts on Thurs, or Wed on leap year).
		// b) it ends on Friday and starts on Thurs (a leap year).
		// In order not to call mktime again, we can work this out from what we already know!
		int lastDay = date->tm_wday + 31 - date->tm_mday;
		if (lastDay == 5) // Last day of the year is Friday
		{
			// How many days in the year?
			int daysInYear = date->tm_yday + 32 - date->tm_mday; // add 32 because yday is 0 to 365
			if (daysInYear < 366)
			{
				// If 365 days in year, then the year started on Friday
				// so there are only 52 weeks, and this is week one of next year.
				week = 1;
			}
		}
		else if (lastDay != 4) // Last day is NOT Thursday
		{
			// This must be the first week of next year
			week = 1;
		}
		// Otherwise we really have 53 weeks!
	}
	return week;
}
 
int getw(         int day,  int month , int year)  
{
	tm date = { 0 };
	date.tm_mday = day;
	date.tm_mon = month - 1;
	date.tm_year = year - 1900;
	// We set time to sometime during the day (midday seems to make sense)
	// so that we don't get problems with daylight saving time.
	date.tm_hour = 12;
	return GetWeek(&date);
}


int main( )
{
     printf( "= %d =\n",  getw(15,5,2013) );

}

    int wdayJan1=  (tm.tm_wday - tm.tm_yday)%7;
    if (wdayJan1<0) wdayJan1 += 7;
    int ydayFirstSunday = (7 - wdayJan1)%7;

    if (tm.tm_yday < ydayFirstSunday) {
        printf ("Last week of previous year\n");
    } else {
        int week_of_year= (tm.tm_yday - ydayFirstSunday)/7;  /* 0-based: 0..52 */
        printf ("It's the #%d week of this year (zero-based)\n", week_of_year);
    }

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
int main( int argc, char * argv[] ){
time_t  t;
        t = time( NULL );
        printf( "%d", ( localtime( &t )->tm_yday )/7 );
        exit( 0 );
}