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1992-11-19
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/* date_int.c: Compute duration between two dates */
#include "date.h"
#define isleap(y) \
((y)%4 == 0 && (y)%100 != 0 || (y)%400 == 0)
static int Dtab [2][13] =
{
{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
{0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};
Date *date_interval(const Date *d1, const Date *d2)
{
static Date result;
int months, days, years, prev_month;
/* Compute the interval - assume d1 precedes d2 */
years = d2->year - d1->year;
months = d2->month - d1->month;
days = d2->day - d1->day;
/* Do obvious corrections (days before months!)
*
* This is a loop in case the previous month is
* February, and days < -28.
*/
prev_month = d2->month - 1;
while (days < 0)
{
/* Borrow from the previous month */
if (prev_month == 0)
prev_month = 12;
--months;
days += Dtab[isleap(d2->year)][prev_month--];
}
if (months < 0)
{
/* Borrow from the previous year */
--years;
months += 12;
}
/* Prepare output */
result.month = months;
result.day = days;
result.year = years;
return &result;
}