① 获取当月开始日期和结束日期

DateTime currentTime = DateTime.Now;
DateTime CurrentMonthStart = DateTime.Parse($"{currentTime.Year}-{currentTime.Month+1}-1");//当月开始日期
int days=System.Threading.Thread.CurrentThread.CurrentUICulture.Calendar.GetDaysInMonth(currentTime.Year, currentTime.Month+1);//当月天数

DateTime CurrentMonthEnd = DateTime.Parse($"{currentTime.Year}-{currentTime.Month+1}-{days}");//当月结束日期

②获取本周开始时间和结束时间

DateTime weekstar = DateTime.Parse(DateTime.Now.AddDays(-(int)DateTime.Now.DayOfWeek + 1).ToString("yyyy-MM-dd 00:00:00"));
DateTime weekend = DateTime.Parse(DateTime.Now.AddDays(7 -(int)DateTime.Now.DayOfWeek).ToString("yyyy-MM-dd 23:59:59"));

③ 获取当月第一天和下个月第一天

 DateTime DateNow = DateTime.Now;
 DateTime DateBegin1 = new DateTime(DateNow.Year, DateNow.Month, 1);//当月第一天
 //获取下个月第一天
 DateTime DateNow1 = DateNow.AddMonths(1);
 DateTime DateBegin2 = new DateTime(DateNow1.Year, DateNow1.Month, 1);//下个月第一天  

④ 计算两个日期之间相差的天数

相关参考网址：https://blog.csdn.net/zht666/article/details/21601403

                      https://blog.csdn.net/e_wsq/article/details/7278760

 TimeSpan ts1 = new TimeSpan(time1.Ticks);
 TimeSpan ts2 = new TimeSpan(time2.Ticks);
 TimeSpan ts = time1.Subtract(time2.Now).Duration();
 int dateDiff = ts.Days;

⑤今天是星期几

1.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" }; 
string week = Day[Convert.ToInt32(DateTime.Now.DayOfWeek.ToString("d"))].ToString(); 
lbDay.Text = week; 

2.
public static string dt; 
public static string week; 
dt = DateTime.Today.DayOfWeek.ToString(); 
switch (dt) 
{ 
case "Monday": 
week = "星期一"; 
break; 
case "Tuesday": 
week = "星期二"; 
break; 
case "Wednesday": 
week = "星期三"; 
break; 
case "Thursday": 
week = "星期四"; 
break; 
case "Friday": 
week = "星期五"; 
break; 
case "Saturday": 
week = "星期六"; 
break; 
case "Sunday": 
week = "星期日"; 
break; 
} 
lbDay.Text = week; 

3.string Week =System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.GetDayName(DateTime.Now.DayOfWeek);

⑥查询某月某周几的日期

string yearandmonth = item.DutyDate.ToString("yyyy-MM");
    int year = item.DutyDate.Year;
    int month = item.DutyDate.Month;
     int d = DateTime.DaysInMonth(year, month);                 //获取指定年月的天数
     if (item.Dutymodel == "周")
       {
        string[] weeks = item.weeks;
       List<string> datelist1 = new List<string>();
     for (int i = 0; i < weeks.Length; i++)
       {
         switch (weeks[i])
          {
         case "周一":
       for (int a = 1; a <= d; a++)
       {
        DateTime dt = new DateTime(year, month, a);
         if (dt.DayOfWeek == DayOfWeek.Monday)
         {   dt.ToShortDateString()//获取的日期
       }  }
                        break;
         case "周二":
            for (int a = 1; a <= d; a++)
               {
                   DateTime dt = new DateTime(year, month, a);
                  if (dt.DayOfWeek == DayOfWeek.Tuesday)
                  {
     }
                      }
           break;
              case "周三":
                for (int a = 1; a <= d; a++)
              {
          DateTime dt = new DateTime(year, month, a);
            if (dt.DayOfWeek == DayOfWeek.Wednesday)
                {
                   }
              }
              break;
            case "周四":
           for (int a = 1; a <= d; a++)
             {
             DateTime dt = new DateTime(year, month, a);
               if (dt.DayOfWeek == DayOfWeek.Thursday)
            {
      
  }  }
             break;
         case "周五":
          for (int a = 1; a <= d; a++)
            {
         DateTime dt = new DateTime(year, month, a);
             if (dt.DayOfWeek == DayOfWeek.Friday)
                {   }
         }
                  break;
             case "周六":
              for (int a = 1; a <= d; a++)
              {
            DateTime dt = new DateTime(year, month, a);
              if (dt.DayOfWeek == DayOfWeek.Saturday)
             { }
           }
           break;
            case "周日":
             for (int a = 1; a <= d; a++)
            {
          DateTime dt = new DateTime(year, month, a);
             if (dt.DayOfWeek == DayOfWeek.Sunday)
        {
 }  }
            break;
               default:
    break;

            }
             }

⑦ 获取某段日期范围内的所有日期

    /// <summary> 
    /// 获取某段日期范围内的所有日期，以数组形式返回  
    /// </summary>  
    /// <param name="dt1">开始日期</param>  
    /// <param name="dt2">结束日期</param>  
    /// <returns></returns>  
    private DateTime[] GetAllDays(DateTime dt1, DateTime dt2)
    {
        List<DateTime> listDays = new List<DateTime>();
        DateTime dtDay = new DateTime();
        for (dtDay = dt1; dtDay.CompareTo(dt2) <= 0; dtDay = dtDay.AddDays(1))
        {
            listDays.Add(dtDay);
        }
        return listDays.ToArray();
    }

⑧判断某个日期是否在某段日期范围内

    /// <summary>
    /// 获取某段日期范围内的所有日期，以字符串形式返回 
    /// </summary>
    /// <param name="startDate">开始日期</param>
    /// <param name="endDate">结束日期</param>
    /// <returns></returns>
    protected string GetDate(DateTime startDate, DateTime endDate)
    {
        string result = string.Empty;
        for (DateTime temp = startDate.ToShortDateString(); temp <= endDate.ToShortDateString(); temp = temp.AddDays(1))
        {
            if (result == string.Empty)
            {
                result = temp.ToString();
            }
            else
            {
                result += "," + temp.ToString();
            }
        }
        return result;
    }

⑨  C#时间类型 DateTime? 赋空值 关于datetime类型的DBNull判断

 public DateTime? SubTime { get; set; }
   //日期类型判断是否为null 用nullable这个泛型结构体
   m.SubTime=dr["subtime"]==DBNull.Value?new Nullable<DateTime>():Convert.ToDateTime(dr["subtime"])

⑩合集

/今天   
DateTime.Now.Date.ToShortDateString();   
//昨天，就是今天的日期减一   
DateTime.Now.AddDays(-1).ToShortDateString();   
//明天，同理，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();   
//如果你还不明白，再看一下中文显示星期几的方法就应该懂了   
//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，同理，一个周是7天，上周就是本周再减去7天，下周也是一样   
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();   
//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,很多人都会说本月的第一天嘛肯定是1号，最后一天就是下个月一号再减一天。当然这是对的   
//一般的写法   
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，不用再解释了吧   
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，很多人都会觉得这里难点，需要写个长长的过程来判断。其实不用的，我们都知道一年四个季度，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();