1 #include "stdio.h"
 2 void main ()
 3 {
 4     int i,Y,M,D,day=0;
 5     int m[13];
 6     printf ("input the date please(year.month.day):");
 7     scanf ("%d %d %d",&Y,&M,&D);
 8     m[0]=0;
 9     m[1]=31;
10     m[3]=31;
11     m[4]=30;
12     m[5]=31;
13     m[6]=30;
14     m[7]=31;
15     m[8]=31;
16     m[9]=30;
17     m[10]=31;
18     m[11]=30;
19     m[12]=31;
20     m[2]=!(Y%4)&&(Y%100)||!(Y%400)?29:28;
21     for (i=0; i<M; i++)
22     {
23         day+=m[i];
24     }
25     day+=D;
26     printf ("It is %dth day .",day);
27 }

这是C语言的一种写法，还有多种，算法思想不同。