C# DateTime类

//今天

DateTime.Now.Date.ToShortDateString();

//昨天，就是今天的日期减一

DateTime.Now.AddDays(-1).ToShortDateString();

//明天，今天的日期加一

DateTime.Now.AddDays(1).ToShortDateString();
//本周(要知道本周的第一天就得先知道今天是星期几，从而得知本周的第一天就是几天前的那一天，要注意的是这里的每一周是从周日始至周六止
DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();
DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek)))).ToShortDateString();


//由于DayOfWeek返回的是数字的星期几，我们要把它转换成汉字方便我们阅读，有些人可能会用switch来一个一个地对照，其实不用那么麻烦的              
string[] Day = new string[] { "星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六"}

Day[Convert.ToInt16(DateTime.Now.DayOfWeek)];


//上周，一个周是7天，上周就是本周再减去7天，下周也是一样  

1 DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7).ToShortDateString(); 
2 DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek))) - 7).ToShortDateString();

//下周

DateTime.Now.AddDays(Convert.ToDouble((0 - Convert.ToInt16(DateTime.Now.DayOfWeek))) + 7).ToShortDateString();
DateTime.Now.AddDays(Convert.ToDouble((6 - Convert.ToInt16(DateTime.Now.DayOfWeek))) + 7).ToShortDateString();

//本月

DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1"; //第一天
DateTime.Parse(DateTime.Now.Year.ToString() + DateTime.Now.Month.ToString() + "1").AddMonths(1).AddDays(-1).ToShortDateString();// 最后一天


//巧用C#里ToString的字符格式化更简便 

DateTime.Now.ToString("yyyy-MM-01");

DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")) .AddMonths(1).AddDays(-1).ToShortDateString()


//上个月，减去一个月份

DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(-1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//下个月，加去一个月份

DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")) .AddMonths(2).AddDays(-1).ToShortDateString();

//7天后

DateTime.Now.Date.ToShortDateString();
DateTime.Now.AddDays(7).ToShortDateString();

//7天前 DateTime.Now.AddDays(-7).ToShortDateString();
DateTime.Now.Date.ToShortDateString();


//本年度，用ToString的字符格式化我们也很容易地算出本年度的第一天和最后一天
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).AddDays(-1).ToShortDateString();


//上年度

DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(-1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddDays(-1).ToShortDateString();

//下年度

DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).ToShortDateString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")) .AddYears(2).AddDays(-1).ToShortDateString();


//本季度，一年四个季度，一个季度三个月
//首先我们先把日期推到本季度第一个月，然后这个月的第一天就是本季度的第一天了

DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");

//同理，本季度的最后一天就是下季度的第一天减一

DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)) .ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//下季度

DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.AddMonths(6 - ((DateTime.Now.Month - 1) % 3)) .ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//上季度

DateTime.Now.AddMonths(-3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)) .ToString("yyyy-MM-01")).AddDays(-1).ToShortDateString();

//如何获得当月有多少天

int m=System.DateTime.DaysInMonth(System.DateTime.Now.Year,System.DateTime.Now.Month);

//获取某月的实际工作日(即不包括周六日)

 int days =getDays(System.DateTime.Now)); 

private int getDays(System.DateTime date1) 

{ 

int m=System.DateTime.DaysInMonth(date1.Year,date1.Month); 

int mm=0; for(int i=1;i<=m;i++)

 { 

System.DateTime date=Convert.ToDateTime(date1.Year+"-"+date1.Month+"-"+i); 

switch (date.DayOfWeek) 

{ 

case System.DayOfWeek.Monday:

 case System.DayOfWeek.Thursday:

 case System.DayOfWeek.Tuesday: 

case System.DayOfWeek.Wednesday: 

case System.DayOfWeek.Friday: 

mm=mm+1; break;

}

} 

return mm; 

}

获得任意两日期之间的有效工作日(不包括周六日)  

DateTime date1=Convert.ToDateTime("2005-10-20"); 

DateTime date2=Convert.ToDateTime("2005-11-01");

 int days =getDays(date1,date2); 

private int getDays(System.DateTime date1,System.DateTime date2) 

{ 

string m=DateDiff(EnumDateCompare.day,date1,date2).ToString("f0");
int mm=0; 

for(int i=0;i<=Convert.ToInt32(m);i++) 

{ 

System.DateTime date=Convert.ToDateTime(date1.AddDays(i)); 

switch (date.DayOfWeek) { 

case System.DayOfWeek.Monday: 

case System.DayOfWeek.Thursday: 

case System.DayOfWeek.Tuesday: 

case System.DayOfWeek.Wednesday: 

case System.DayOfWeek.Friday:

 mm=mm+1; break; }            

} 

return mm;

 }

获得本周的周六和周日

#region ConvertDateToWeek

public static void ConvertDateToWeek(DateTime date,out DateTime firstdate,out DateTime lastdate)

{

DateTime first=System.DateTime.Now;

DateTime last=System.DateTime.Now;

switch (date.DayOfWeek)

{

case System.DayOfWeek.Monday:

first=date.AddDays(-1);

last=date.AddDays(5);

break;

case System.DayOfWeek.Tuesday:

first=date.AddDays(-2);

last=date.AddDays(4);

break;

case System.DayOfWeek.Wednesday:

first=date.AddDays(-3);

last=date.AddDays(3);

break;

case System.DayOfWeek.Thursday:

first=date.AddDays(-4);

last=date.AddDays(2);

break;

case System.DayOfWeek.Friday:

first=date.AddDays(-5);

last=date.AddDays(1);

break;

case System.DayOfWeek.Saturday:

first=date.AddDays(-6);

last=date;

break;

case System.DayOfWeek.Sunday:

first=date;

last=date.AddDays(6);

break;

}

firstdate=first;

lastdate=last;

}

#endregion

//调用

DateTime firstdate=System.DateTime.Now;

DateTime lastdate=System.DateTime.Now;

ConvertDateToWeek(date,out firstdate,out lastdate);

获得当前日期是该年度的第几周

DateTime dt=Convert.ToDateTime("2006-05-01");

int weeks=dt.DayOfYear/7+1;

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原文地址：https://www.cnblogs.com/GeneralKING/p/2610075.html