void Main()
{
    DateTime dt = DateTime.Now;  //当前时间
    
    DateTime startWeek = dt.AddDays(1 - Convert.ToInt32(dt.DayOfWeek.ToString("d")));  //本周周一
    DateTime endWeek = startWeek.AddDays(6);//本周周日
    
    DateTime startMonth = dt.AddDays(1-dt.Day);//本月月初
    DateTime endMonth = startMonth.AddMonths(1).AddDays(-1);//本月月末
    
    DateTime startQuarter = dt.AddMonths(0 - (dt.Month - 1) % 3).AddDays(1 - dt.Day);  //本季度初  
    DateTime endQuarter = startQuarter.AddMonths(3).AddDays(-1);  //本季度末
    
    DateTime startYear = new DateTime(dt.Year, 1, 1);  //本年年初  
    DateTime endYear = new DateTime(dt.Year, 12, 31);  //本年年末
    
    var startQuarter2 = startYear.AddMonths(3);//2季度初
    var endQuarter1 = startQuarter2.AddDays(-1);//1季度末
    var startQuarter3 = startYear.AddMonths(6);//3季度初
    var endQuarter2 = startQuarter3.AddDays(-1);//2季度末
    var startQuarter4 = startYear.AddMonths(9);//4季度初
    var endQuarter3 = startQuarter4.AddDays(-1);//3季度末
    
    Console.WriteLine("本周周一：" + startWeek.ToString("yyyy-MM-dd"));
    Console.WriteLine("本周周末：" + endWeek);
    
    Console.WriteLine("\r\n本月月初：" + startMonth);
    Console.WriteLine("本月月末：" + endMonth);
    
    Console.WriteLine("\r\n本季度初：" + startQuarter);
    Console.WriteLine("本季度末：" + endQuarter);
    
    Console.WriteLine("\r\n本年年初：" + startYear);
    Console.WriteLine("本年年末：" + endYear);
    
    Console.WriteLine("\r\n一季度初：" + startYear);
    Console.WriteLine("一季度末：" + endQuarter1);
    
    Console.WriteLine("\r\n二季度初：" + startQuarter);
    Console.WriteLine("二季度末：" + endQuarter2);
    
    Console.WriteLine("\r\n三季度初：" + startQuarter);
    Console.WriteLine("三季度末：" + endQuarter3);
    
    Console.WriteLine("\r\n四季度初：" + startQuarter);
    Console.WriteLine("四季度末：" + endYear);
    
    Console.WriteLine();
    
    System.DateTime startTime = TimeZone.CurrentTimeZone.ToLocalTime( new System.DateTime(1970, 1, 1)); // 当地时区
    long timeStamp = (long)(DateTime.Now - startTime).TotalMilliseconds; // 相差毫秒数

    Console.WriteLine("时间戳：" + timeStamp);

}

本周周一：2019-08-12
本周周末：2019-08-18 17:44:42

本月月初：2019-08-01 17:44:42
本月月末：2019-08-31 17:44:42

本季度初：2019-07-01 17:44:42
本季度末：2019-09-30 17:44:42

本年年初：2019-01-01 00:00:00
本年年末：2019-12-31 00:00:00

一季度初：2019-01-01 00:00:00
一季度末：2019-03-31 00:00:00

二季度初：2019-07-01 17:44:42
二季度末：2019-06-30 00:00:00

三季度初：2019-07-01 17:44:42
三季度末：2019-09-30 00:00:00

四季度初：2019-07-01 17:44:42
四季度末：2019-12-31 00:00:00

时间戳：1565862282444