日期

日期和字符串一样，都不能改变自身的值

System.DateTime d = System.DateTime.Now;
2

System.Console.WriteLine(d.AddDays(1));
3

System.Console.WriteLine(d.AddHours(1));
4

System.Console.WriteLine(d.AddMilliseconds(1));
5

System.Console.WriteLine(d.AddMinutes(1));
6

System.Console.WriteLine(d.AddMonths(1));
7

System.Console.WriteLine(d.AddYears(1));
8

System.Console.WriteLine(d);//d没有改变值
9

d = d.AddYears(1);//需要重新赋值

一些关于日期的算法要牢记

System.Console.WriteLine(d.AddDays(-(d.Day-1)));//本月第一天
2

System.Console.WriteLine(d.AddDays(-d.Day));//上月最后一天
3

System.Console.WriteLine(d.AddDays(-(int)d.DayOfWeek));//本周第一天

ok，我们来看一个例子：

System.DateTime dt = new System.DateTime(1949, 10, 1);
2

System.Console.WriteLine(dt.ToLongDateString());
3

System.DateTime d = System.DateTime.Now;
4

int toDay = d.Day;
5

int thisMonth = d.Month;
6

System.Console.WriteLine("当前日期：{0}", d.Date);
7

d = d.AddDays(-(toDay - 1));
8

System.Console.WriteLine("该月第一天：{0}", d.Date);
9

do
11

{
12

string toDayFlag = "";
13

if (d.Day == toDay)
14

{
15

toDayFlag = "*";
16

}
17

System.Console.WriteLine("{0}:{1}{2}", d.Day, d.DayOfWeek, toDayFlag);
18

d = d.AddDays(1);
19

}
20

while (d.Month == thisMonth);

上面例子输出本月的所有日期和星期，并在当日标注*

那么开始巩固知识：

设一企业合同约定每月5号发工资，请输出该企业今年实际每月发工资的日期（若5号是周六或周日，则工资提前到周五发）

System.DateTime dt = System.DateTime.Now;
2

dt = dt.AddDays(-(dt.DayOfYear - 1));
3

System.Console.WriteLine("该年第一天：{0}", dt.Date);
4

dt = dt.AddDays(4);
5

int iDay = 0;
6

System.Console.WriteLine("定位到第一个月的5号：{0}", dt.Date);
7

do
8

{
9

if (dt.Month != 5 && dt.Month != 10)
10

{
11

if (dt.DayOfWeek == System.DayOfWeek.Saturday)
12

{ iDay = -1; }
13

if (dt.DayOfWeek == System.DayOfWeek.Sunday)
14

{ iDay = -2; }
15

}
16

else
17

{ iDay = -5; }
18

System.Console.WriteLine("{0}月的发薪水日期是{1}",
19

dt.Month, dt.AddDays(iDay).Date);
20

dt = dt.AddMonths(1);
21

iDay = 0;
22

}
23

while (dt.Year == System.DateTime.Now.Year);

设每年9.1开学，若9.1不是星期一，则要求开学日期改为9月的第一个星期一，请输出从1980-2004年的每年开学日期

//方案1
2

System.DateTime dt = new System.DateTime(1949, 9, 1);
3

System.DateTime tmpdt;
4

do
5

{
6

if (dt.DayOfWeek == System.DayOfWeek.Monday)
7

{ tmpdt = dt; }
8

else
9

{
10

if (dt.DayOfWeek == System.DayOfWeek.Sunday)
11

{ tmpdt = dt.AddDays(1); }
12

else
13

{ tmpdt = dt.AddDays(7 - (int)dt.DayOfWeek + 1); }
14

}
15

System.Console.WriteLine("{0}年的9.1是{1}", dt.Year, dt.DayOfWeek);
16

System.Console.WriteLine("所以实际开学日期是：{0}", tmpdt.Date);
17

System.Console.WriteLine("------------------------");
18

dt = dt.AddYears(1);
19

}
20

while (dt.Year <= System.DateTime.Now.Year);
21

//方案2
24

System.DateTime dt = new System.DateTime(1949, 9, 1);
25

System.DateTime tmpdt;
26

do
27

{
28

if ((int)dt.DayOfWeek < 2)
29

{ tmpdt = dt.AddDays(1 - (int)dt.DayOfWeek); }
30

else
31

{
32

tmpdt = dt.AddDays(7 - (int)dt.DayOfWeek + 1);
33

}
34

System.Console.WriteLine("{0}年的9.1是{1}", dt.Year, (int)dt.DayOfWeek);
35

System.Console.WriteLine("所以实际开学日期是：{0}", tmpdt.Date);
36

System.Console.WriteLine("------------------------");
37

dt = dt.AddYears(1);
38

}
39

while (dt.Year <= System.DateTime.Now.Year);

注意方案2的第29行，使用了01互换的方法，简化了周日和周一的计算

计算给定日期的年的每个月的周数

System.DateTime tmpdt = new DateTime(year, 1, 1);
2

int tmpMonth = tmpdt.Month;//记录当前月
3

int[] weeks = new int[12];//每个元素存放每月的星期数量
4

do
5

{
6

do
7

{//如果是周六就加1
8

if (tmpdt.DayOfWeek == System.DayOfWeek.Saturday)
9

{
10

weeks[tmpMonth - 1] = weeks[tmpMonth - 1] + 1;
11

}
12

tmpdt = tmpdt.AddDays(1);//下一天
13

}
14

while (tmpdt.Month == tmpMonth);//月循环
15

if (tmpdt.Day == 1 && tmpdt.DayOfWeek != System.DayOfWeek.Sunday)
16

{//如果每月的第一天不是周日，则上月再加一周
17

weeks[tmpMonth - 1] = weeks[tmpMonth - 1] + 1;
18

}
19

tmpMonth = tmpdt.Month;
20

}
21

while (tmpdt.Year == year);//年循环
22

for (int i = 0; i <= weeks.Length - 1; i++)
23

{
24

System.Console.WriteLine("{0}年 {1}月有{2}周", year, i + 1, weeks[i]);
25

}