日期和字符串一样，都不能改变自身的值
1
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
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
1
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
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：
1
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
10
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);
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
10
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）
1
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);
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
else17
{ 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
1
//
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
22
23
//
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);
//
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
else9
{10
if (dt.DayOfWeek == System.DayOfWeek.Sunday)11
{ tmpdt = dt.AddDays(1); }12
else13
{ 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
22
23
//
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
else31
{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
1
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 }
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
do7
{//如果是周六就加18
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
}
本文转自shyleoking 51CTO博客，原文链接:http://blog.51cto.com/shyleoking/806905
