DateTime类的用法很多，这里把自己用到的总结一下

using System;

namespace Moxell.Library
{
 /// <summary>
 /// DateTime 的摘要说明。
 /// </summary>
 public class DateTimeLibrary
 {
  public DateTimeLibrary()
  {
  }

 

        // 返回指定日期是一年中的第几周
   
  public static int GetWeek(DateTime dt)
  {
   int Week = 0;
   System.Globalization.CultureInfo norwCulture = System.Globalization.CultureInfo.CreateSpecificCulture("no");
   System.Globalization.Calendar cal = norwCulture.Calendar;
   
   Week = cal.GetWeekOfYear(dt, System.Globalization.CalendarWeekRule.FirstFourDayWeek, System.DayOfWeek.Sunday);

   return Week;
  }

        // 当月的第一天，接受日期参数
  public static DateTime GetTheMonthFirstDay(DateTime theDate)
  {   
   return new DateTime(theDate.Year,theDate.Month,1);
  }

        // 当月的第一天，接受字符串参数

  public static DateTime GetTheMonthFirstDay(string YearMonth)
  {
   DateTime firstDay = new DateTime(Convert.ToInt32(YearMonth.Substring(0,4)),Convert.ToInt32(YearMonth.Substring(4,2)),1);
   return firstDay;
  }
        //当月的最后一天日期
  public static DateTime GetTheMonthLastDay(DateTime theDate)
  {
   return new DateTime(theDate.Year,theDate.Month,DateTime.DaysInMonth(theDate.Year,theDate.Month));
  }
        //返回一个指定日期年，月和今天是本月第几天的日期值
  public static DateTime GetTheMonthToDay(DateTime theDate)
  {
   return new DateTime(theDate.Year,theDate.Month,DateTime.Today.Day);
  }
        //根据需要更的用法，可以自己理解一下
  public static DateTime GetFromDate(DateTime theDate)
  {
   DateTime yesterday  = theDate.AddDays(-1);
   return GetTheMonthFirstDay(yesterday);
  }
        //根据需要更的用法，可以自己理解一下
  public static DateTime GetToDate(DateTime theDate)
  {
   DateTime yesterday  = theDate.AddDays(-1);
   return yesterday;
  }
        //返回下月的第一天，可要仔细看了
  public static DateTime GetTheCurrDDlSelectDate(string CurrDDlDate)
  {
   
   string CurrDateMonth= CurrDDlDate.Substring(4,2);
   string CurrDateYear=CurrDDlDate.Substring(0,4);

   int ConvMonth=Convert.ToInt32(CurrDateMonth);
   int NeedConvMonth=ConvMonth+1;
   
   string NeedRebackMonth=Convert.ToString(NeedConvMonth);

   if(NeedRebackMonth.Length==1)
    NeedRebackMonth="0"+NeedRebackMonth+"-01";
   else
    NeedRebackMonth=NeedRebackMonth+"-01";
   
   string CurrNeedDate=CurrDateYear+"-"+NeedRebackMonth;
   return Convert.ToDateTime(CurrNeedDate);
  }
        //下面这几个自己理解哈
  public static DateTime GetDateByString(string stringDate)
  {
   DateTime ToDay = DateTime.Today;
   DateTime resultDate = DateTime.MinValue;
   switch (stringDate.ToLower())
   {
    case "today":
     resultDate = ToDay;
     break;
    case "oneweek"://最近一周
     resultDate = ToDay.AddDays(-7);
     break;
    case "onemonth"://最近一月
     resultDate = ToDay.AddMonths(-1);
     break;
    case "threemonth"://最近三月
     resultDate = ToDay.AddMonths(-2);
     break;
    case "sixmonth"://最近六月
     resultDate = ToDay.AddMonths(-3);
     break;
    case "halfyear"://最近半年
     resultDate = ToDay.AddMonths(-6);
     break;
    case "oneyear"://最近一年
     resultDate = ToDay.AddYears(-1);
     break;
    case "twoyear"://最近二年
     resultDate = ToDay.AddYears(-1);
     break;
    case "threeyear"://最近三年
     resultDate = ToDay.AddYears(-1);
     break;
    default:
     resultDate = ToDay.AddYears(-100);
     break;
   }
   return resultDate;
  }

  public static DateTime GetBeginDateByString(string stringDate)
  {
   DateTime BeginDate = DateTime.MinValue;

   if (stringDate.ToLower()=="all")
   {
    string Today="2006-01-01";
    BeginDate =Convert.ToDateTime(Today);
   }
   else
   {
    BeginDate = new DateTime(Convert.ToInt32(stringDate.Substring(0,4)),Convert.ToInt32(stringDate.Substring(4,2)),1);
   }
   return BeginDate;
  }

  public static DateTime GetEndDateByString(string stringDate)
  {
   DateTime EndDate = DateTime.MinValue;

   if (stringDate.ToLower()=="all")
   {
    DateTime today= DateTime.Today;
    EndDate=GetTheMonthLastDay(today);
   
   }
   else
   {
    EndDate = new DateTime(Convert.ToInt32(stringDate.Substring(0,4)),Convert.ToInt32(stringDate.Substring(4,2)),1);
    EndDate = GetTheMonthLastDay(EndDate);
   }
   return EndDate;
  }

 

 }
}